What usually happens with all credit and debit cards is the Hotel or
DCL, seek authorisation for a certain amount when you hand them your card at check in.
Each WDW resort has a different policy I was pre authorised to $500 at the All Stars and $1500 at the Contemporary.
This authorisation is good for a period of time say a week or 28 days and eats into your Card limit.
During that time if you reach your ''Room'' limit of authorisation the Hotel or DCL passes the payment through and seeks another authorisation, if this failed your credit could be stopped.
So if you spend $500and thats your ship limit, it debits your account and resets the onboard account to nil.
Best advice is to
Tell the credit card company in advance where and what you are doing, it avoids declines if they think your card is stolen and used abroad.
Have a couple of cards use one for authorisations and use the other for day to day purchases such as shopping onshore etc.
Note the authorisation may use up your normal card limit and other payments could be declined if that are still live, even if you didnt use it fully.
Hope this helps.