How do i use packet markers

Hi im trying to learn how to use packet markers. in my last post Displaying radio stations on the Touchshield slide - #3 by system - Displays - Arduino Forum they to use packets but i have no clue what they are or how to use them. So i need help on how to use use them the these codes

Touchshield:

PImage timedis;
PImage menubut;
PImage batbar;
PImage arrowL;
PImage arrowR;
PImage volButt;
PImage button;
int volume;
char out[3];
unsigned int Xstore; //setup temporary variable 
unsigned int Ystore; //setup temporary variable
void setup(){
  Serial.begin(2400);
  //open(FlashTransfer);
  background(0);
  arrowL = loadImage("arrowL.bmp");
  arrowR = loadImage("arrowR.bmp");
  volButt = loadImage("volButt.bmp");
  timedis = loadImage("timedis.bmp");
  menubut = loadImage("menubut.bmp");
  batbar = loadImage("batbar.bmp");
  button = loadImage("button.bmp");
  image(timedis,260,0);
  image(menubut,0,0);
  stroke(100);
  fill(100);
  image(batbar,136,0);
  rect(140,3,37,9);
  triangle(50,135,70,120,70,150);
  triangle(215,135,195,120,195,150);
  noFill();
  //rect(50,120,20,30);
  //rect(195,120,20,30);
  image(button,45,200);
  image(button,110,200);
  image(button,175,200);
  image(volButt,275,105);
  stroke(153);
  text("VOL",276,90,15);
  //text("15",287,170,11);
  //text("7",292,90,11);
  text("SEEK",110,90,15);
  //text("94.5",110,135,15);
  text("1",70,215,11);
  text("2",135,215,11);
  text("3",200,215,11);
  text("+",293,126,15);
  text("-",293,218,15);
}

void loop(){

  gettouch();
  if(mouseX != Xstore || mouseY != Ystore){
  if(mouseX > 45 & mouseX < 95 & mouseY > 200 & mouseY < 230){
    Serial.print('a');
    stroke(0,170,80);
    text("1",70,215,11);
    stroke(153);
    text("2",135,215,11);
    text("3",200,215,11);
    stroke(0);
    fill(0);
    rect(75,120,115,30);
    stroke(153);
    //dtostrf(channel,2,0,out);
    //text(out,120,135,15);
  }
  else if(mouseX > 110 & mouseX < 160 & mouseY > 200 & mouseY < 230){
    Serial.print('b');
    stroke(0,170,80);
    text("2",135,215,11);
    stroke(153);
    text("1",70,215,11);
    text("3",200,215,11);
    stroke(0);
    fill(0);
    rect(75,120,115,30);
    stroke(153);
    //dtostrf(channel,2,0,out);
    //text(out,120,135,15);
  }
  else if(mouseX > 175 & mouseX < 225 & mouseY > 200 & mouseY < 230){
    Serial.print('c');
    stroke(0,170,80);
    text("3",200,215,11);
    stroke(153);
    text("1",70,215,11);
    text("2",135,215,11);
    stroke(0);
    fill(0);
    rect(75,120,115,30);
    stroke(153);
    //dtostrf(channel,2,0,out);
    //text(out,120,135,15);
  }
  
  if(mouseX > 195 & mouseX < 215 & mouseY > 120 & mouseY < 150){
    Serial.print('u');
    stroke(153);
    text("1",70,215,11);
    text("2",135,215,11);
    text("3",200,215,11);
    stroke(0,170,80);
    fill(0,170,80);
    triangle(215,135,195,120,195,150);
    delay(100);
    stroke(100);
    fill(100);
    triangle(215,135,195,120,195,150);
    stroke(0);
    fill(0);
    rect(75,120,115,30);
    stroke(153);
    //dtostrf(channel,2,0,out);
    //text(out,120,135,15);
  }
  if(mouseX > 50 & mouseX < 70 & mouseY > 120 & mouseY < 150){
    Serial.print('d');
    stroke(153);
    text("1",70,215,11);
    text("2",135,215,11);
    text("3",200,215,11);
    stroke(0,170,80);
    fill(0,170,80);
    triangle(50,135,70,120,70,150);
    delay(100);
    stroke(100);
    fill(100);
    triangle(50,135,70,120,70,150);
    stroke(0);
    fill(0);
    rect(75,120,115,30);
    stroke(153);
    //dtostrf(channel,2,0,out);
    //text(out,120,135,15);
  }
  if(mouseX > 280 & mouseX < 306 & mouseY > 119 & mouseY < 159){
    Serial.print('+');
    stroke(0,170,80);
    text("+",293,126,15);
    delay(100);
    stroke(0);
    fill(0);
    rect(280,160,26,17);
    volume++;
    stroke(153);
    text("+",293,126,15);
  }
  if(mouseX > 280 & mouseX < 306 & mouseY > 180 & mouseY < 225){
    Serial.print('-');
    stroke(0,170,80);
    text("-",293,218,15);
    delay(100);
     stroke(0);
    fill(0);
    rect(280,160,26,17);
    volume--;
    stroke(153);
    text("-",293,218,15);
  }
  Xstore = mouseX; 
  Ystore = mouseY; 
 }
 if(volume >= 15){
   volume=15;
 }
 if(volume <= 0){
   volume=0;
 }
 if(volume < 10){
   dtostrf(volume,2,0,out);
    text(out,281,170,11);
 }else{
    dtostrf(volume,2,0,out);
    text(out,287,170,11);
  }
char charIn = 0;
byte i = 0;
char stringIn[32] = "";

while(Serial.available()){
charIn = Serial.read();
stringIn[i] = charIn;
i += 1;
}
  // Clear screen and display the serial buffer as text
  if (stringIn[0]){
    fill(0);
    stroke(0);
   rect(75,120,115,30);
   stroke(153);
   text(stringIn,120,135,15);
  }
 }

Arduino:

#include <Si4703_Breakout.h>
#include <Wire.h>
#include <SoftwareSerial.h>
#define RXpin 3
#define TXpin 2
char radByte;
int resetPin = 7;
int SDIO = A4;
int SCLK = A5;

Si4703_Breakout radio(resetPin, SDIO, SCLK);
int channel;
int volume;
char rdsBuffer[10];
SoftwareSerial touchSerial = SoftwareSerial(RXpin, TXpin);

void setup()
{
  touchSerial.begin(2400);
  /*Serial.println("\n\nSi4703_Breakout Test Sketch");
  Serial.println("===========================");  
  Serial.println("a b     Favourite stations");
  Serial.println("+ -     Volume (max 15)");
  Serial.println("u d     Seek up / down");
  Serial.println("r       Listen for RDS Data (15 sec timeout)");
  Serial.println("Send me a command letter.");*/
  

  radio.powerOn();
  radio.setVolume(0);
}

void loop(){
  radByte = touchSerial.read();

    if (radByte == 'u') 
    {
      channel = radio.seekUp();
      displayStation();
    } 
    else if (radByte == 'd') 
    {
      channel = radio.seekDown();
      displayStation();
    } 
    else if (radByte == '+') 
    {
      volume ++;
      if (volume == 16) volume = 15;
      radio.setVolume(volume);
    } 
    else if (radByte == '-') 
    {
      volume --;
      if (volume < 0) volume = 0;
      radio.setVolume(volume);
    } 
    else if (radByte == 'a')
    {
      channel = 945; // Jamn 94.5 FM
      radio.setChannel(channel);
      displayStation();
    }
    else if (radByte == 'b')
    {
      channel = 969; // 
      radio.setChannel(channel);
      displayStation();
    }
    else if (radByte == 'c')
    {
      channel = 1033; // 
      radio.setChannel(channel);
      displayStation();
    }
}
    void displayStation(){
    touchSerial.print(channel);
    delay(1000);
    }

ps. the touchshield and arduino are coded seperatly