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