Sheets and plugins are basically the same thing, as sheets are just plugins that are flagged for printing or exporting. However, it’s often more convenient to refer to those as print sheets or export sheets, since that’s how they’re used, rather than a Unified View plugin, which shows up as a little box on screen.
Sheets and plugins may appear here as beta releases before they become available through the package manager (inside GCA, get there using the Check for Updates option on the Help menu). If you choose to try out a beta plugin, you’ll want to manually remove it after it becomes available through the package manager, and then install it through that; that way automatically checking for updates will pick it up correctly.
To manually add a plugin to GCA5: Unzip the file into your
Documents/GURPS Character Assistant 5/plugins/ folder, so that your
/plugins/ folder has a subfolder of
/PLUGINNAME/ and within that are a bunch of files. GCA5 should then pick it up automatically and make it available to you. For example, Spring Bandit Sheet should be in your GCA5 documents folder at
/plugins/SpringBanditSheet/ and all the plugin files would be in there.