Do you mean the "exchange/source" (i.e. normally N for NYSE, P for ARCA, etc.) or the "condition"? If you're talking about the "condition", T is the indicator for "Form-T", which is basically any trade that occurs outside regular market hours (09:30-16:00 ET). I've never seen a T in the "exchange/source" field. ITS and block-crossing networks seem to print S.