What does "MDL-01" stand for ?

[John D.]

Using maps online I'm looking at a building that is listed as follows:

OCC Description

APT 4-UNT MDL-01

I'm attempting to make sense of what that MDL-01 means.  

 think the answer is somewhere in the link below but I very well could be wrong... and even if the answer is in there I'm not clear of what it means / significance. 

https://www.mass.gov/files/documents/2018/04/10/521cmr9_0.pdf

Thanks AGAIN.. in advance!!

-JD