tiers/tierce

I am having a lot of trouble submitting questions!
In brief, can you please explain again about "un tier", a masculine (noun) and une tierce, a feminine noun or is it an adjective?
You use tiers (masculine) or tierce (feminine) instead of troisième when expressing a fraction, but une tierce de personne comme un temon can't be a third of a person.

Bonjour Paul !

The expression is une tierce personne as in a third party in English. It's usually used in a legal context for example.

I cannot really think of other cases when we use tierce, as tiers is the one you used when expressing a fraction, hence in most cases:
un tiers, deux tiers...
Un tiers du monde    -> a third of the world

I hope that's helpful!
Bonne journée !

Merci Aurélie.