- and + represent the polarity of the given system, as I understand it (not quite the same as antenna polarity or electrical, so that's all I can say).
The BS DATA means it's transmitting pure data (DATA) from the base station (BS) to the field units which could be text messages or other related info - it can also indicate a private/encryption transmission for a given group on DMR as I understand how it works. DMRDecode would be able to identify encrypted or non-encrypted comms by denoting it as "private," iirc. I don't use DMRDecode much anymore, it's nice to have but for me it doesn't actually do much in my usage.
The BS VCX means it's transmitting voice channel audio (VCX) from the base station (BS) to the field units and that is what DSD/DSD+ is capable of decoding.
It's entirely possible to have something like BS DATA on slot 1 while having BS VCX on slot 2 since DMR/MOTOTRBO is a 2-slot TDMA based form of digital communication meaning it can send out both types of signals at the same time across the same frequency. Pretty neat stuff, actually.
DSD+ detects the given polarity automagically for such signals so it's not something you have to bother with while the older DSD (iirc, I haven't used it in quite some time) required you to specify positive or negative DMR polarity or else the decodes would be complete gibberish, more or less.