What word does this adverb describe?

[Quark]

In this sentence, what does the word "mostly" describe?

I wake up with an aching down my right arm and a severe tingling and burning in my right hand, mostly in the middle finger.

Thanks for any help.

[Azariah]

I'd say that it means "most of the pain (maybe 90% of the pain) is in the middle finger."

[Julie]

Mostly modifies aching. You could substitute, primarily or for the most part.

[xiphiasatsi]

This 'mostly' means that compared to other fingers,your middle finger hurts more.