SYNC Clause and Slack Bytes

Binary items that are aligned at natural storage boundaries might perform better than items that are not aligned. COBOL provides two alignment related features.

SYNC clause
This clause ensures that the item is placed at the next natural boundary, if applicable. The COBOL compiler achieves that by inserting slack bytes before the item.
Top-level items
The compiler always places top-level copybook items at a double-word (8-bytes) boundary. For example, if your copybook contains more than one item at the top level, then there might be a gap in front of the second and the following items: 
01 TOP-LEVEL1.
      02 ....
01 TOP-LEVEL2.
      02 ...
01 TOP-LEVEL3.
      02 ...
In the above, TOP-LEVEL2 and TOP-LEVEL3 are always pushed to the following 8-byte boundary, which might result in unused bytes in front of these items.