(Re-)Integrating Tcl support into Doxygen questions

I am looking to re-integrating Tcl support into Doxygen.  I have forked the 
github repo of Doxygen and have started working on creating the parsers for 
Tcl, using pyscanner.* and pycode.* as a model and working on the tclscanner.* 
files from doxygen 1.8.13 as a reference.

I have some questions:

doxygen 1.8.* uses a single parser/scanner for each language, but doxygen 
1.9.* uses two: an Outline Parser and a Code Parser.  What is the difference? 
I guess the short answer is the files are parsed in two passes and each of the 
passes of processed differently.  Is there documentation on what each pass 
needs to do?  

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