I find the MBTA website info on passes very confusing.
The way the site reads a Monthly LinkPass (subway, bus) is not usable on Commuter Rail Zone 1a, or the Charlestown Ferry.
But a Commuter Rail Zone 1a Pass, or a Charlestown Ferry monthly pass (both same price as a LinkPass) are usable on subway and busses.
Not a very user friendly way to run a system.
I would assume it's deliberately this way so that people who seek out the 1A pass are actually using the commuter rail. It might influence how much Keolis gets, etc.