The mode parameter to the Parse* functions is a set of flags (or 0). They control the amount of source code parsed and other optional parser functionality.
const (
PackageClauseOnly uint = 1 << iota // parsing stops after package clause
ImportsOnly // parsing stops after import declarations
ParseComments // parse comments and add them to AST
Trace // print a trace of parsed productions
DeclarationErrors // report declaration errors
)