In uGet settings, plugins, I see the option called 'Plug-in matching order'.
Curl or aria2 can be selected as the active program that runs the download with two of the options; but what the other two do (curl+aria2, aria2+curl) ?
These are download engines for uGet.
You can use CURL or aria2 as the backend engine to do the download. These plugins provide different features so you can pick which one you want...or you can choose an order to use the plugins.
For example, if one plugin is faster for you then you can use that one and if you attempt to do a feature that it doesnt support it will move onto the next plugin to accomplish the task.