If checked then, when issuing an "accurev show ... streams" command, the plugin will use the "-s streamname" option to restrict the output to just the stream in question, look at the result to determine the basis stream and then repeat for that stream's parent etc until the full ancestry is obtained. If not checked, when the plugin needs to know about the stream it is using, it'll simply get all of the streams from the server (which can involve a lot of data).

This is best enabled on accurev servers with a lot of streams, snapshots, workspaces etc.
When there are relatively few streams and snapshots aren't used, it is typically quicker to get all the streams that the server knows about in one single command. When there are a lot of them, it's often faster to send multiple commands that get the stream you want, and then its parent, and its parent etc until you reach the top of the chain.