Lockbox Input file clarification on Record Group Id

Hi All ,
I am working with lock box implemention and i need little more clarification ,

Point # 1) what is the importance of the Record Group ID right from Payment Details , customer Details , and item references. , i can assume that one payment may have multiple reference but unable to figure out the senarios for payments and customer details
Point # 2) what kind of possible scenarios for payment details
Point # 3) if you have any sample file with record group id can you pls send me


1DEMO   LOCKBX LOCKB 00001000000000145410602052013082007
2DEMO   LOCKBX LOCKB LOCKB               40010201021
30000000002026210006900000000000000002000000000145410600000400002121121121           1231123110
3000000000202621000690000000000100000000100050000000202062013USD
30000000002026210006900001000002Demo100799     0000000047850002062013EFTUSD
3000000000202621000690000100000300002                                  123456789
30000000002026210006900001000007I 002-579188                                       
30000000002026210006900002000002Demo100710     0000000052200002062013EFTUSD
3000000000202621000690000200000300002                                  123456789
30000000002026210006900002000007I 002-579263

3000000000202621000700000000000100000000045360600000208082012USD
30000000002026210007000001000002Demo100798     0000000021131202062013EFTUSD