What is the actual definition of a "rental door?"

[Mustafa Abdulali]

Newbie question here. As I research REI I keep coming across the term "per door" but it seems to have different definitions. Let's assume we are talking about a typical 3/2 single family.

In one article it is suggested to allocate $100 per door/month for maintenance for each door (not sure if that only applies to multi-units or single family).  Does that mean I should allocate $500 or $300 per month for maintenance?

In a craigslist post a seller talks about "rental door" and defines that as the number of front doors (the ad is for several triplexes).  In this case he has a Triplexes and states "3 rental doors" but each unit is a 2/1.  Should he state "9 rental doors?"  Would my maintenance cost on the triplex be $300 per month (front door) or would it be $900 per month ($100 for each bedroom and bathroom door in the building)?

I want to make sure that I use a valid rule of thumb when evaluating property.  

Thanks!