If a data item description contains the SYNCHRONIZED clause, and word-storage mode is enabled, the position of that item within the computer storage is aligned so that the right-hand (least-significant) end is on a natural boundary of the computer's storage. Extra character positions (bytes) of computer storage are reserved adjacent to synchronized items to achieve this alignment; these bytes, known as padding bytes or implicit FILLER bytes, are normally inaccessible to the computer program except as part of a group item.
Each elementary data item that is described as SYNCHRONIZED is aligned to the natural storage boundary that corresponds to its data item storage assignment (according to Table 5 in the topic Computer Memory Natural Boundaries above). Thus, in word-storage mode, a numeric data item with a PICTURE description of S9(5) would be assigned 4 bytes of storage (being 1 padding byte and 3 data bytes). If SYNCHRONIZED was specified, it would be aligned to the next nearest 4-byte boundary (that is, with the total (4-byte) storage assignment aligned such that the number of bytes from the beginning of the record containing that item to the left-hand (most-significant) end of that item was a multiple of four). If the previous item does not end on a 4-byte boundary, implicit FILLER assignments are necessary to achieve this.
Other such implicit FILLER bytes can be generated by the use of SYNCHRONIZED items in non-elementary data descriptions containing an OCCURS clause (see the topic The OCCURS Clause). This is because further bytes may need to be reserved for each group item occurrence in order that the second or subsequent occurrences have the same alignment to the natural boundaries of the computer storage as did the first occurrence.