documentclass{article}
usepackage{fontspec}% xelatex
begin{document}
section{Macro Name Scanning}
defmacro{123}
count1=macro
4
This is a register: thecount1. Notice that the first 4 was consumed by the TeX scanner as expected.

section{Register Value Scanning}
count1=42
advancecount1 by1000The < The textit{T} in textit{The} should have been consumed by the TeX scanner before being typeset. Where does textbackslash relax come into play here?% Why is the T not consumed by the number scanner (catcode 12)
end{document}