I have just stumbled on one more reference which suggests that using hypo clearing agents extends archival capacity of the fixer from its, otherwise, measly equivalent of 10 8x10" sheets per 1 l. In Tim Rudman's "Toning Book" (2010, ISBN 1-902538-23-4) on page 156, chapter "Archival permanence", section "Hypo-Clearing Agents", 5th paragraph reads:

Hypo-clearing agents play an important part in the archival processing cycle. Firstly, their use extends the archival standard fixer capacity of a single-bath fixer by about a factor of 4.