The Special Interest Groups Dictionary is an optional grouping table where associations that use SIGs can define the groups formed by your members in order to pursue significant long-term activities based on shared interests, knowledge or a field of study. The examples in the next screen shot reflect technology groups, but they can be defined for a wide variety of interests. 
Since this is a grouping table, you create MasterCodes under each GroupCode for sub-groups, as seen in the next screen shot for the Programming (PROG) GroupCode.
You can set up as many GroupCodes and MasterCodes as you need. If you do not have sub-groups then you can create the MasterCode using the same value as the GroupCode, but you must create at least one MasterCode. For example, if the Programming GroupCode did not have different sub-groups as show above, then you would create a single MasterCode of Programming (PROG) under the GroupCode of Programming (PROG).