Yes DEC or "decimal", is the number to program for the talkgroup ID number.
With ID Scan, you are scanning only those talkgroup ID numbers that you have programmed into your scanner. (However if you lock the talkgroup out, the scanner skips over the talkgroup - what us older heads may also call "channel")
In the ID Search mode, you are "hearing" all the talkgroups. (Again, you can lock out talkgroups that are discovered on search and they will be skipped over).
If there are specific talkgroups you want to listen to, the ID Scan is the way to go. If you want to hear anything or everything, ID Search is your choice.