There is a difference between a pressure cooker and a pressure canner. Pressure cookers are smaller than canners and also have different features. Pressure cookers are intended for producing foods for immediate consumption, so they don't have the same type of gauges found on canners, which are meant for processing foods for long term storage.
There are some products on the market that claim to be either-or. These need to be approached with caution in the canning department, because they do not meet the minimum requirements for pressure canning set by the USDA. This is a food safety issue. You will surely hear people defend the use of pressure cookers for canning, saying they have been using them for years with no problem. I'd like to hear from the ones who didn't do so well. Unfortunately, botulism is a deadly poison.
While pressure cookers are not recommended for use as pressure canners, it is possible to use a pressure canner as a pressure cooker. The main problem would be that the greater size makes them inconvenient for preparing anything but very large volumes of food. My recommendation is to put both the pressure cooker and a canner on your wish list. I love my pressure cookers. At the present time I have 3 Presto and 2 Fagor, in sizes ranging from 4qt to 10qt.
Information on capacity: When a pressure cooker description give the capacity --- 6qt, for instance --- that is the amount of liquid it will hold to the brim. However, the maximum amount of food that can be cooked under pressure is much less than that. The manual will tell you 3/4 full (4.5 qts) for foods that do not foam, or not more than 1/2 full (3 qts) for foods that do foam. So when making your purchase, you will want to take this into consideration. The same is true for pressure canners. If the description says 23 qts, for instance, that does not mean you can process 23 quarts in one batch. Its the amount of liquid the pan will hold to the brim.