Teamcenter Unified Architecture What is user exits Server exits Custom exits?

In simple terms Use exit is Already written function which we will modify as per requirement like we are changing logic of Part Revision i.e Default Part Revision is starts with “A”  we can 
 modify it to “NR”  that means  we are Modifying Already available function.

In simple terms Server exits is New function which we want to be executed on the server side when some event from Client (User Interface like Rich Client) happens.

 Eg: If we have provided Some Menu option in Rich Client to user and we want on Click event of it, some Functionality on the server side to be executed in that case we go for writing Server exit.

In simple terms Custom exits is Way to modify User Exit and Server exit to avoid unintentional changes in the code which affect the other functionality.