From the Mass DOR:

The List of Corporations Subject to Taxation in Massachusetts (also called the Corporations Book) is published online each spring and assists local assessors in determining whether a particular entity owning personal property as of January 1 of the publication year is entitled to certain property tax exemptions as a corporation.

 
The Annual Certification of Entity Tax Status is essential to determining or verifying your company’s status.
 
What entities must file the Annual Certification of Entity Tax Status?
Existing and recently formed (1) corporations, and (2) other business entities, including limited liability companies (LLCs), partnerships, business trusts, certain entities organized in foreign countries and other unincorporated entities, that are classified as corporations for federal and Massachusetts tax purposes, must file a certification if they have personal property in a Massachusetts city or town as of January 1, 2018 and want to ensure they are listed in the 2018 Corporations Book as an entity eligible for specific local property tax exemptions as a corporation. See DOR Directive 12-05.

Business entities that (1) have changed their entity tax status since their 2017 certification or (2) are no longer treated as corporations for federal and Massachusetts tax purposes, but were listed in the 2017 Corporations Book, also must file.

Note that an entity operating in Massachusetts that qualifies as an S corporation subsidiary ("QSUB"), or a member of a unitary business operating in Massachusetts, may qualify for a personal property tax exemption and should file separately. 

What entities are not required to file?
Non-profit corporations or entities that may be eligible for local property tax exemptions under G.L. c. 59, § 5 as charitable, religious, veteran or other organizations are not required to file the Annual Certification of Entity Tax Status. Local boards of assessors make those exemption decisions based on information provided in exemption applications and annual returns the organizations file with them.

Sole proprietors are also not required to file the Annual Certification of Entity Status.
 
How to file
Complete the certification on MassTaxConnect under “Business” where you will see “Submit annual certification of entity tax status.”
 
More information
Annual Certification of Entity Tax Status webpage
Frequently asked questions available on MassTaxConnect