Transaction Codes in SAP are actually direct paths for the various objects such as Dictionary and Repository Objects.
Actually SE means SYSTEM ENGINEERING
and SM means SYSTEM MAINTANENCE.
Examples are SE11(ABAP Dictionary), SE38(ABAP Editor), etc....
and SM30(Table Maintanence Generator) etc...
Generally SExx are ABAP side T Codes,i.e. They are for development purposes, and SMxx are for the Basis Side ,i.e. these T Codes are generally for maintanence purposes.
Hope this resolves your query.
<b>Reward all the helpful answers.</b>
SE means System Environment
SM means System Management
As such there is no logic behind using SE or SM. But looking at the transactions one can say that all the SM transactions are for Monitoring purpose while all SE transactions are Executable transaction.
Go SE11 and give the table name "TSTC" and click on display, u can get all the tcodes.
Goto Transaction SE84 --> Other Objects --> Transactions
--> In the Transaction Code enter 'RS', 'SE', 'SM*' those are all the BW & R3 transactions used.
Assign points if it helps.