subscriber
A person who signs up to receive messages from a particular client and/or their list(s).


Inactive subscribers fall into 2 categories  permanent inactive (in which the only way they can become active again is for you to edit their profile or the user themselves does this) and temporary inactive (which can be the result of a temporary block, like a spam block)

With temporary inactive subscribers, they are automatically readded to your database after a given period of time (for spam blocks, it is 48 hours). Given this, when you upload new data that say contains addresses that are currently inactive in the system, such addresses will not be added to your database.

This is to protect you from sending emails to subscribers that have already requested not to be contacted again.