Only Knights who owned land (and there for people) had symbols.
If you were a regular Infantry soldier, like a peasant, you would wear your lord's symbol on your tunic.
That way in the middle of battle if you lost the part of the army you were supposed to be in 
You could look at your chest see the symbol and look for the flag with the same symbol. 

If you were a knight, but only lived in your lord's household and didn't own any land 
Then you would wear his symbol or "device".
They had these devices because many could not read, even knights and nobles.
This system of devices intertwines greatly with the feudal system, 
And where knights fit in with the feudal system.
I have found a great book which explains in detail the feudal system, 
Why it was created, and the things a couple people don't realize about it.
It is called 
Medieval Knights by Trevor Cairns, for those interested... 
