As you advance in skill your pocket has everything to do with personal preference and playing style. There are three main types of pockets you can have. You can have a high, mid, or low pocket.
The most common and recommended pocket for all levels is a mid. to low pocket.
The advantage of having a high pocket is that it has the quickest release of the three pocket types. This does not mean that the ball will go faster out of your stick, it means that it will take less time for the ball to leave the pocket. This happens because the distance of where the ball sits to the top of the stick is shorter than a high or mid. pocket. The disadvantages of having a high pocket is that the ball will come out of the stick much easier when cradling up and down( with your stick perpindicular to the ground) with a high pocket. Also It is common for a high pocket to have inconsistent throwing patterns and because of this a high pocket is only recommended for very advanced players.
A Mid. Pocket is just as the name suggests, a middle of the road selection. A mid. pocket gives a good release and a good throwing pattern, but the release will not be as good as the high pocket and the throwing pattern and ball retention will not be as good as the low pocket. But, the Mid pocket is the best all around pocket in regards to ball retention and throwing pattern.
The advantage of having a low pocket is that you will have the most consistent throwing pattern out of all the pocket types. Having a consistent throwing pattern is the most important thing in a stick. Also the ball will stay in the stick much better when cradling with the stick straight up and down. The disadvantage with a low pocket is that there will be a slower release and typically the ball will come out of the top of the stick easier than the other two pockets.