The Green Line is often classed as Light Rail. Its operating manual was used as the basis for Edmonton's, which has then been used as the basis for other LRT lines.
So, due to its history, the Boston Green Line:
-- has a street-running streetcar segment shared with traffic (the E branch) (plus several other non-revenue street-running tracks)
-- has substantial street-median sections derived from old streetcar exclusive ROW, with streetcar-type station spacing (these are the B and C branches)
-- has a branch with much longer station spacing which was converted from a former mainline railway and its former suburban stations... which were originally stations in entirely separate cities before the metropolitan area engulfed them, so it's arguably an interurban route (this is the D branch)
-- has the first subway tunnel and subway station in the United States (in downtown Boston)
-- has a substantial elevated section (on the bridge to Cambridge and the new construction there)
-- has two new branches which are grade-separated, next to old railroad mainlines, with urban-rail type station spacing (the GLX extension project)
-- has sections run on line-of-sight like buses (though this is going to change)
-- has sections run on full signalling
So it's got everything: streetcar, interurban, subway, elevated, depends on which part of the system you're on. You're right that when the LRT term was introduced, it was meant to describe this level of flexibility; I guess the Green Line really was the model for LRT.