How change files are applied for pdftex/pdfetex?

============================= pdftex ============================= 

2) pdftex.ch is generated from
    ./tie -c pdftex.ch pdftex.web \
    $(srcdir)/pdftexdir/tex.ch0 \
    $(srcdir)/tex.ch \
    $(srcdir)/pdftexdir/tex.ch1 \
    $(srcdir)/pdftexdir/tex.pch

Input:
    pdftex.web: 
        pdftex without system-dependent changes

    pdftexdir/tex.ch0:
        % Change file to assist in creating the web2c-specific change file.
        % This one comes first, resolves inconsistencies between the
        % generated pdftex.web and tex.ch

        change the pdftex banner to tex banner, so tex.ch can be applied

    tex.ch: 
        the main web2c-specific (or other system-dependent) change file

    pdftexdir/pdftex.ch:

        % Change file to assist in creating the web2c-specific change file.
        % This one resolves inconsistencies between tex.ch and tex.pch.

        change the tex banner to pdftex banner (reverse tex.ch0)

Output:
    pdftex.ch (not pdftexdir/pdftex.ch):
        additional & system-dependent changes
