Credit Card Blocking
Credit card blocking occurs when a merchant contacts your credit card company with an estimate of what your total bill will be and "blocks" that amount even though he isn't charging your card for the total amount at that time and might not ever charge your card for that amount.
An example of a situation where credit card blocking occurs is when you use a credit card to register with a hotel or car rental service. They call in an estimate of what your total bill might be to your credit card issuer and have that amount set aside or blocked for possible future use. This way you can't stiff them on the final bill later on.
While an amount has been blocked it cannot be used to make other purchases. So, if you leave your hotel room to go shopping, you might find your credit card declined on the basis that you have reached your credit limit.
Some credit card companies will let you go over your limit and then access an "over the limit" fee on your next statement. These fees are heavy and you want to avoid them if at all possible.
For the above reason, it is not a good idea to use a credit card that you have nearly maxed out when you travel.