So I understand through reading and asking questions that the Arduino Uno digital output pin should not acceed 40 mA output but the 5v pin can. This obviously is useful for powering relays with transistors I have found. So why is it that that the digital output pin can't source more than 40 mA but the 5v can? And what is the limit on what the 5v pin can source and why would that not be listed on the Arduino Uno SMD? Just a couple questions from someone new to this. Thanks guys!
The I/O pin 5V comes via a tiny transistor on the processor chip, the 5V pin comes straight off the on-board regulator.
The limit on 5V should be kept well sub-500mA
output pin should not acceed 40 mA output
I think you mean exceed.
and why would that not be listed on the Arduino Uno SMD
Because it depends on the input voltage. This determines how much heat the regulator generates. See:-
http://www.thebox.myzen.co.uk/Tutorial/Power.html
and
http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html