A sectioned index sequential set is a single physical file with multiple logical roots.~Block 1 is the section table and looks similar to a typical root table except that it is the entry point to the multiple logical roots. Typically, the root at block 1 of a nonsectioned set or subset is a single root and never has more than TABLESIZE entries. However, for a sectioned set or subset, the section table can actually span multiple blocks, even though it is still considered the main root.

If the section table exceeds the standard TABLESIZE value, blocks are linked together to form a single-root section table. This is all controlled by a single bit in block 0 (zero). If the file is sectioned, bit [16:1] of the table control word in block 1 is on, which indicates that block 1 is a section table as opposed to a root table for a nonsectioned set or subset.

The following figure shows the file layout of a logical single-file sectioned set. All sections reside in a single file. The table starts from block 1. Root tables of each section follow the section table.