The Subindex flag (>) works as a subindex entry flag only if you
    have issued an .INDEX or .ENTRY command. Subindex entries marked
    with this flag are collected and alphabetized below the primary
    entry to which they refer. The Subindex flag indicates that the
    next word or phrase will be placed on the following line of the
    index, indented two characters to the right of the preceding
    entry.

1  –  Default

    Recognition is turned on only within .INDEX and .ENTRY commands.