TBox[a, b, ...] produces a RowBox[{a,b, ...}] where a, b, ... are boxed in TraditionalForm.
TBox[a, b, ...]
RowBox[{a,b, ...}]
a, b, ...
TraditionalForm
TBox is used internally by FeynCalc to produce the typeset output in TraditionalForm.
TBox
Overview
TBox[a + b] % // DisplayForm
FormBox[RowBox[{a,+,b}],TraditionalForm]\text{FormBox}[\text{RowBox}[\{\text{a},+,\text{b}\}],\text{TraditionalForm}]FormBox[RowBox[{a,+,b}],TraditionalForm]
a+ba+ba+b