If you wonder how the number is generated for access certain records in CRM. Here's a reference for you.
Access Right Values:
Read = 1
Write = 2
Append = 4
AppendTo = 16
Create = 16
Delete = 65536
Share = 262144
Assign = 524288
So if you want to grant read and write access, then just add up the read and write value. (e.g. Read (1) + Write (2) = ReadWrite (3) )