2 family used as 1 family - what does this mean?

[Matt Fitzgerald]

I'm looking at a multifamily property in Boston, MA and it says "This home is a legal 2-family but used as a single family with a 1 bedroom in law apartment."  When I look the property up on the city of Boston assessing site has it classified as "101 (Residential Property / SINGLE FAM DWELLING)"

I'd like to purchase this property and rent it out as two apartments.  Is there anything I should be aware of, or look out for?  What does it mean its a legal 2-family, but used as a single family? 

Thanks!