\textlf{} for light font weight – how common?
An example for HelveticaNeue-Light
\documentclass{article}
\usepackage{fontspec}
\setmainfont[
ItalicFont = HelveticaNeue-Italic,
BoldFont = HelveticaNeue-Bold,
BoldItalicFont = HelveticaNeue-BoldItalic]{HelveticaNeue}
\newfontfamily\NHLight[
ItalicFont = HelveticaNeue-LightItalic,
BoldFont = HelveticaNeue-UltraLight,
BoldItalicFont = HelveticaNeue-UltraLightItalic]{HelveticaNeue-Light}
\newcommand\textrmlf[1]{{\NHLight#1}}
\newcommand\textitlf[1]{{\NHLight\itshape#1}}
\let\textbflf\textrm
\newcommand\textulf[1]{{\NHLight\bfseries#1}}
\newcommand\textuitlf[1]{{\NHLight\bfseries\itshape#1}}
\begin{document}
\textrm{foo} \textbf{bar} \textit{baz} \textbf{\textit{foobar}} \par
\textrmlf{foo} \textbflf{bar} \textulf{bar} \textitlf{baz} \textuitlf{foobar}
\end{document}
in a texlive 2014, updated this morning I get the following from grep (with my annotation in comment at the end of each line):
$ grep "[^a-z]textlf[^a-z]" */*
allrunes/allrunes.sty:\DeclareTextFontCommand{\textlf}{\lfseries}
baskervaldx/Baskervaldx.sty:\DeclareRobustCommand{\textlf}[1]{% lining figures
fbb/fbb.sty:\DeclareRobustCommand{\textlf}[1]{% lining figures
heuristica/heuristica.sty:\DeclareRobustCommand{\textlf}[1]{% lining figures
typeface/typeface.sty: \PassOptionsToPackage{textlf}{MinionPro}% lining figures
xcharter/XCharter.sty:\def\textlf#1{{\lfstyle #1}}% lining figures
$ pwd
/usr/local/texlive/2014/texmf-dist/tex/latex
So it seems that the convention is that \textlf
where used in all other packages denotes lining figures.