Calc stands for "Calculated". When IDMS stores a record it calculates a target page based on a logical/symbolic key using hash alogorithm. So to get a calc record you need to supply the key value and say OBTAIN which will fetch the record from database.
Another thing to note is that OBTAIN is Equivalent to FIND + GET. Let me explain you what find and get mean.
FIND - will check the database for occurance of the record in the DB.
GET - will fetch the record for which currency has been established by previous FIND command in to Call area.
OBTAIN will do both establish currency onthe record and retrieve it to the Call area.
Hope this helps...
for remaining types of records and stuff I would suggest you to go thru IDMS manual or books..