mjhjmtu

When using document command arguments for case distinctions I routinely have been using the str_case:nn function, which works fine as is evident from the output of the myStrCase command. When attempting to use the tl_case:Nn function instead, by loading the argument into a _tl type variable, things go miserably wrong. The variable seems to be loaded with the correct value, and is then flagged as correct as far as the tl_case:Nn function is concerned, but the corresponding code is not executed. Why?

documentclassarticle
%=======================
usepackage[check-declarations,log-functions]expl3
usepackagexparse
%-----------------------
ExplSyntaxOn
tl_new:N l_rn_Aux_tl
NewDocumentCommandmyStrCaseOemphm
 
 str_case:nnTF #1
 
 emphtextit#2
 Emphtextbftextit#2
 ulineunderline#2
 
 
 !~ERROR:~parameter~#1~=~#1~invalid
 % myStrCase
NewDocumentCommandmyTlCaseOemphm
 
 tl_set:Nx l_rn_Aux_tl #1
checking~what~was~passed:~l_rn_Aux_tl \
 tl_case:NnTF l_rn_Aux_tl
 
 emph textit#2
 Emph textbftextit#2
 uline underline#2
 
 parameter~#1~=~#1~valid
 !~ERROR:~parameter~#1~=~#1~invalid
 % myTlCase
ExplSyntaxOff
%-----------------------
begindocument
subsectionusing str_case:
 myStrCaseHi There\
 myStrCase[emph]Hi There\
 myStrCase[Emph]Hi There\
 myStrCase[uline]Hi There\
 myStrCase[xxx]Hi There
subsectionusing tl_case:
 myTlCase[emph]Hi There\
 myTlCase[xxx]Hi There
enddocument

The documentation (interface3) already contains your answer:

tl_case:NnTF <test token list variable>

<token list variable case1> <code case1>
<token list variable case2> <code case2>
...
<token list variable casen> <code casen>

<true code>
<false code>

You will find "token list variable" and not "token list" therein. Hence you are asked to provide one token (a variable/N-type argument). So to use this construction you would need to save your options into variables and use these within tl_case.