OCCURS Clause cannot be used in 01, 66, 77 and 88 levels.
There are many reasons for that:
1. 01 Level items are usually record level items. OCCURS clause is used for specifying multiple occurrences of fields and NOT the records.
2. If OCCURS clause specified in 01 level, you cannot perform search and search all operations in an efficient manner.
3. In COBOL, Table is nothing but a ?Structure of Array?. So you have to specify the array (occurs) within a structure (01 level).
4. In COBOL59, 01 and 77 levels are aligned on a double word boundary. So if you have multiple occurrences for an item declared in 01/77 level, each item should start in a Word Boundary. So CODASYL didn't allow to use OCCURS clause in 01 & 77 level to avoid unnecessary slack bytes.