I think I may have answered my own question:
"3. When Qualifying Service
Is On Vessels Of Five GT Or Less.
46 CFR 10.422(e) provides that master or mate licenses issued to applicants presenting
obtained aboard vessels of five gross tons or less will be limited to
vessels of no more than 25 gross tons. This provision is independent of the other
provisions of section 10.422. In other words, an individual who never serves on a vessel
larger than five gross tons will not be allowed to raise their tonnage limit. In order to raise
that tonnage limitation, a master or mate must present evidence of service obtained aboard
vessels of greater than five gross tons as follows (see 46 CFR 10.422(b)(4)):
a. An additional six months service on vessels over five gross tons will allow the
tonnage to be increased to 50 gross tons; and
b. Another six months service on vessels over five gross tons (a total of twelve months
service on vessels over five gross tons) will allow the tonnage limit to be increased to
100 gross tons."