What is suggestion type in DQM in SAP CRM?

I found a code related to DQM.

According to that code if suggestion type is equal to 'FD' then a particular action will be performed.Now, my problem is I dont' know what 'FD' or 'SC' or 'RANG' mean.

I have also checked in SPRO and found that various suggestion list ids are maintained like 'PRSC', 'FD' , 'SC' ,'RANG' , ST, UNIT etc.I have also gone for a F4 help but nothing is there in F4 help.

Can anyone tell me what exactly is suggestion type and how a suggestion type is determined?