RE: Understands Dynamics GP 2013 Tables and fields in detail.
What I usually do on any version is load out the latest SDK for GP. You can find it on your media though you might also find it on Customersource (if they have ever fixed it).
GP2015\Installation Files\Install Media\MDGP2015_R2_DVD_ENUS\Tools\SDK\Dynamics GP\ In it is a self extracting program (small) that will load a set of documentation for you.
It has diagrams of tables and relationships, version changes but also has a Table Integration set of documents (word docs) and I find I spend 90% of my time in that area when I'm looking for answers similar to questions you are asking.
The table integration documentation pre dates IM and eConnect and so is a detailed description of work tables you could write records to in the bad old days. It does not describe open or historical tables (such as RM30201) but you can often extrapolate values back to the field descriptions of the work tables and it's helpful to answer questions such as yours above.
From the RMTRX word doc for values in RM_Sales_Work:
Field descriptions: (Required fields appear in bold and are underlined)
Document Type: Stores the value from the selected position of the Document Type drop-down list. This is stored in the work table to allow for future reordering of the Document Type list
Field values:
1 = Invoice
2 = Finance Charge
3 = Miscellaneous Charge
4 = Return
5 = Credit Memo
6 = Payment
RM Document Type-All: Stores the actual document type.
Field values:
0 = Reserved for Balance Carried Forward Records
1 = Sales / Invoice
2 = Reserved for Scheduled Payments
3 = Debit Memos
4 = Finance Charges
5 = Service Repairs
6 = Warranties
7 = Credit Memos
8 = Return
9 = Payments
RM Document Number
-WORK: This is the 17-character editable field for entering document numbers that are unique for this document type. The next default document number for each document type is stored in the RM_Document_Setup table. When the document is created, this field's value should also be stored in the Document Number field.
Etc.....