How do and's and or's work in filters?

I need to use multiple filters at the same time. What are the priorities of the and's and or's? 

i.e. If I use the following filters:

customer ID = A

or customer ID = B

and campaign ID <> X

does the campaign ID filter act as a subset of the customer ID = B filter or does it apply to both of the previous filters?  

Does it work like:

(customer ID = A) or (customer ID = B and campaign ID <> X)

or like 

(customer ID = A or customer ID = B) and (campaign ID <> X)

Thanks!