[lex.phases]/1.3

The source file is decomposed into preprocessing tokens ([lex.pptoken]) and sequences of whitespace characters (including comments). A source file shall not end in a partial preprocessing token or in a partial comment. Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of whitespace characters other than new-line is retained or replaced by one space character is unspecified. The process of dividing a source file's characters into preprocessing tokens is context-dependent.

After this phase, the sequence of the tokens in a source file should be in compliance with that specified by preprocessing-file that is defined in [cpp.pre], it is unused in the current draft but it is useful to interpret what is a valid sequence of preprocessing tokens in a source file. For instance

// a.h

// b.cpp
#include "a.h"
module ;
export module X;

b.cpp is an invalid source file after translation phase 3 even though a.h is empty. A valid processing file is either group<opt> or module-file but not both.

[lex.phases]/1.7

Whitespace characters separating tokens are no longer significant. Each preprocessing token is converted into a token ([lex.token]). The resulting tokens are syntactically and semantically analyzed and translated as a translation unit.

translation-unit is defined in [basic.link] and is unused in the current draft. However, it is useful to interpret what the grammar or resulting tokens should be in a valid translation unit, translation-unit specifies that and is useful here.