the "S" appears when it's detecting a digital signal of some kind (say, a DMR "rest" channel)... the "T" appears when it's successfully decoding it (either a control channel or voice channel).
Sort of.
The 'S' icon appears in place of the 'T' icon when the scanner is looking for "trunking" data but hasn't yet found it, but RF squelch is open. It applies to all trunking modes, not just digital.
It is replaced by the 'T' icon when the scanner starts decoding "trunking" data.
When scanning any kind of trunked system, the 'S' icon will appear, if only briefly, every time the scanner tunes to a candidate control channel and gets RF squelch. Normally (i.e. it really is a control channel and the scanner is able to decode data), it will be immediately replaced by 'T'.
If you see the 'S' icon more than just briefly, it means:
* RF squelch level set too low,
* tuned frequency isn't a control channel, or
* unable to reliably decode data
The 'S' icon can also appear with conventional DMR channels, because it's looking for "trunking" data.
The primary intent of the 'S' icon is to tell the user that he likely has RF squelch set too low. In some trunked systems (e.g. LTR) or when you have lots of convention DMR channels programmed, where the scanner has to cycle through all programmed frequencies, constantly seeing that 'S' icon means the system / channels are being scanned
very slowly.