Tiny question google can't really answer

Projects Programming
5

Robin2:
If you want to change the name of a variable you must change all occurrences of it in the program. My guess is that you did not change the line that defines the variable

char data = 0;

...R

Originally there wasn't a thing that defined the variable. it was just " data = Serial.read"
My main question is why doesn't data need to be defined but any other word does?
I can easily put " char doggo = Serial.read" and it works fine but that's not the issue