You need to read -- cover to cover, no matter how long it takes -- http://www.redbooks.ibm.com/redbooks/pdfs/sg246105.pdf
which is a link to the VSAM Demystified
An RRDS consists of a set of slots for records. These slots are sequentially numbered 1, 2, 3, 4, ... and go up to however many slots you defined the RRDS to contain (hence the NUMBERED option in the IDCAMS DEFINE command to indicate you are defining an RRDS). If you place something in slot #2, there is a record in that slot. If you do not place anything in slot #2, there is still a slot #2. In fact, without deleting the entire RRDS you cannot get rid of slot #2 -- it must always exist.
The 0001, 0003, 0004, and so forth -- if they are part of the record then they are placed in the slot just like every other byte of data in the record. If they are the slot number ,then they tell which slot to place the record in. If they are part of the record, there is no relationship between the slot number and that part of the record -- you could put the record containing 0003 in slot 2, or you could put the record containing 0918 in slot 2 ... it all depends upon how you load the data into the RRDS.