Since at least 1989, 46 CFR 15.901(a) has read as follows:
“An individual holding a merchant mariner credential (MMC) endorsed as mate or pilot of inspected self-propelled vessels of 200 gross registered tons (GRT) or more is authorized to serve as master on inspected self- propelled vessels of less than 100 GRT within any restrictions on the MMC, without further endorsement.”
While the language and intent seem clear, for several years now mariners holding an inspected license (Mate 500, 1600, 3rd Mate, 2nd Mate) not only had to make application for the Master 100 GT but had to test for it as well. When offered jobs requiring the Master 100, these mariners would apply for the credential, sometimes with explicit reference to the CFR citation above, only to be told that they would have to take the full examination for Master 100.
However the Coast Guard has just released a new policy letter (CG-MMC Policy Letter No. 03-21) removing the requirement for the examination and recognizing that the inspected Mate 200 GT or over covers the licensing requirement for mariners operating as Master on vessels less than 100 GT. So, for example, a Mate 500 GT Near Coastal may act as a Master 100 NC, with or without the Master 100 GT endorsement on the license. A Mate 1600 Inland may act as a Master 100 Inland. To have the endorsement placed on a license, a mariner will still have to make an application and presumably pay the required fees, but no testing.
Note that the Mate 200 does not have this implicit authority as technically the 200 GT credential reads “Mate Less than 200 GT.”