DEC is the decimal equivalent of a talkgroup (think of clients in a client server world) used in a trunk system, which is usually stored in hexadecimal (base 16).
Generally talkgroups have no relationship to frequencies. Some talkgroups are repeated on conventional frequencies (such as 16 in Anne Arundel county repeated on 154.01). However the nature of trunking would be that a single talkgroup's usage can jump from frequency to frequency.
I would start by reading the overview section of the following - stick to the basics and tutorial article for now (anything in blue and/or underlined is a link...)