A.2.2 Types of Input Syntax
This appendix describes the standard input syntax used by Mathematica. This input syntax is the one used by default in InputForm and StandardForm. You can modify the syntax by making definitions for MakeExpression[expr, form].
Options can be set to specify what form of input should be accepted by a particular cell in a notebook or from a particular stream.
The input syntax in TraditionalForm, for example, is different from that in InputForm and StandardForm.
In general, what input syntax does is to determine how a particular string or collection of boxes should be interpreted as an expression. When boxes are set up, say with the notebook front end, there can be hidden InterpretationBox or TagBox objects which modify the interpretation of the boxes.