COMP is binary data. COMP items are native binary items (rough equivalent to an int in C). Arithmetic with this kind of data is normally very fast. Use COMP for subscripts and line counters. Use COMP for items that are calculated frequently and moved to DISPLAY-NUMERIC items infrequently (because to move a COMP item to a display numeric one, COBOL generally converts it to packed then to display). For binary data, the field will be 2 bytes long if the picture is S9(01) - S9(04). It will be 4 bytes long if S9(05) - S9(09). And 8 bytes long if S9(10) - S9(18) COMP-1 and COMP-2 are floating point. COMP-3 is packed. COMP-3 is a good choice for running totals, sub-totals and similar data items. Arithmetic is reasonably fast and conversion to display is fairly fast as well. Like COMP items, these should not be in external files that may need to be shared on another platform. COMP-3 fields should always be signed and have an odd number of digits for best performance. (e.g. S9(3) COMP-3 requires 2 bytes; 3 nibbles for the digits and one for the sign. S9(4) requires 3 bytes; 4 nibbles for the digits, one for the sign, and one extra that the compiler must clear whenever the field is changed) SYNC is an abbreviation for SYNCHRONIZED. It is used with binary data to align it on a proper address.
Use of SYNC gives faster access at the cost of storage.