One Term = 1 search term tracked for 1 URL on 1 search engine.
Every URL/Term/Search-Engine combination counts as 1.
Every Local combination (i.e: different city) or a Mobile combination (differnt mobile device) counts as 1.
The terms limit of a plan is Total.
For example, The bronze plan's limit is 300. With this capacity you can have 6 URLs X 50 Terms each.
Unlimited URLs means that there is no terms restriction per URL,
So the actual URL number is unlimited up to the terms capacity in case of tracking 1 term per 1 url.