Can please make Search in this forum, there are so many info available for your questions.
I'm giving here short description about the things you mentioned.
In general, making better use of memory at the time internal representation of calculation or computation we use COMP Variables since they occupy less space depends upon the comp type we use.
COMP: It's a binary storage format, usuall occupy n/2 bytes.
COMP-1: It's internal Single Precision Floating data-item, and it's 4 bytes long
COMP-2:It's internal Double Precision Floating data-item, and it's 8 bytes long
COMP-3:It's Packed decimal storage format, and this is most efficient form of storage, it occupies n/2 + 1 bytes
There are so many info about COMP data-item , make a search.
This is used to align the COMP field in a more specific boundary area such that to make Internal arithmetic operation more efficient.
SYNC Clause is used with COMP,COMP-1,COMP-2 items.