The Plus addition to the name is used in Europe to signify the tool comes in a systainer. So when you buy an ETS 150/3, you get only the tool in a cardboard box, and when you buy the ETS 150/3 Plus, you get the tool in a systainer. Obviously the systainer adds extra functionality and value, hence the Plus.
As far as I know, in the USA you can only buy the tool in a systainer. So the Plus has little meaning there. Yet sometimes they call it Plus and other times not. It's a bit inconsistent.
Unfortuntaly this inconsistency can lead to some confusion, and sometimes US dealers don't know what the Plus means either and they think that the Plus means Set. In Europe the Set suffix means the tool comes with some extra accessories, like for instance the extra chucks for the T15+3 or the rail for the TS55. So dealers accidentally call the Set version Plus on their websites, and the customer also becomes confused.
So in short an example with the TS55
TS55 = saw only
TS55 Plus = saw + systainer
TS55 Set = saw + systainer + 1.40m rail
In Europe you can buy most tools as a Plus or a non-Plus version. Only some also come as a Set.