What does the column “Number of positions/contacts” and “Internal Switches” mean?
The number of positions/contacts parameter is how many termination points there are compared to amount of physical connections on the inside of the barrel. So, a connector like the one below here is a 3 Conductor, 5 Contact has 5 connections to the board and 3 connections that are made internally.
The internal switch parameter basically calls out if the connector has internal switches or not. What is the importance of this switch and why would I use it or need to know about it? To be honest it can be in there and not be used. The most likely scenario to use this internal switch is when you want to know something is plugged in. For example, if you plug headphones into your audio jack on a PC that physical switch tells the circuit something is plugged in and the audio signal should come here now instead of through your speakers or other source. Here are a couple examples of what the schematics of these look like on a typical datasheet.
Here is an example of what that looks like internally on the connector.
You can see that when a plug would be inserted here that the contact at the top with the spring would move and make a new connection.