Question from someone who is confused

Hey there!

Over here in germany, NASCAR is not a very big thing. Formula 1 or other Racing-classes get way more attention. Therefore it's hard to get some information about Nascar.

Correct me if I am wrong:

There are chartered teams, who have guaranteed spots for the race and just do the qualification to determine the starting-position.
There are non-chartered teams, which have to qualify.
The race has spots for 40 drivers.

My question "should" be kind of simple:
If I counted right, the chartered teams for the current season already line up the field of 40 drivers. So how is it possible for a non-chartered driver/team to actually qualify for a race, when all spots are already taken?

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Sorry if my english is not that good. I still hope you understand what I mean. Clarfication for my question is appreciated!

Thanks!