If I may....
If we are talking about "pressure points" in the context of Traditional Chinese Medicine (TCM), the there is no difference between a "pressure point" and a "cavity" in most general usages. The concept of a point in TCM is a cavity.
Look at it this way, when most folks use the term "jing", they are using it incorrectly. For instance, "fa-jing" is incorrect...although it is now generally accepted. Correct terminology would be "fa-jin" (I believe my friend Scott has demonstrated the correct usage on numerous occasions, but it is past most of the people using it incorrectly). The same is true here. What "we" are calling a point are, in fact, a cavity.