When it comes to budget or spending money - you would say "there is no cap" meaning their is no limit on what can be spent or aquired 

A Hat is also called "cap" because you wear it ONTOP of your head, which is the highest part of your person - so when you say there is "no cap" it means there is no top or limit - if there IS a cap, there is a limit 

example: the cap on salary this month is $20,000 (salary can't be more than $20,000)

example: there is NO cap on salary this month (salary can be whatever you need it to be)

Context? 'Cap' has multiple meanings.