ST

ST01 Transaction Set Identifier Code i.e. 997 for a Functional Acknowledgment

ST02 Transaction Set Control Number

AK1

AK101 Functional Identifier Code i.e. if you are acknowledging a SI, send SI

AK102 Group Control Number i.e. the assigned number originated and maintained by the sender

AK2

AK201 Transaction set identifier code i.e. if you are FA an 858 send 858

AK202 Transaction set control number

And AK5 was most interesting to me

AK501 Transaction Set Acknowledgment Code

A Accepted

E Accepted But Errors Were Noted

R Rejected

AK501 Transaction Set Syntax Error Code

Code indicating error found based on the syntax editing of a transaction set

1 Transaction Set Not Supported
2 Transaction Set Trailer Missing
3 Transaction Set Control Number in Header and Trailer Do Not Match
4 Number of Included Segments Does Not Match Actual Count
5 One or More Segments in Error
6 Missing or Invalid Transaction Set Identifier
7 Missing or Invalid Transaction Set Control Number

And for the more in depth PDF download here http://sites.google.com/site/mellerbeck/Home/997_4010.pdf?attredirects=0&d=1