Actually I got lucky just now:

\definefontalternative[md]

\definefontalternative[mc]


Works just fine. Thank you for your suggestion Henning!

On Wed, May 25, 2022 at 7:35 PM Stefan Nedeljkovic <tsar.srb@gmail.com>
wrote:

> mtxrun --script fonts --list --all --pattern=Custodia
> identifier               familyname   fontname                 filename
>                    subfont   instances
>
> custodiabold             custodia     custodiabold
> Custodia-Bold.ttf
> custodiabolditalic       custodia     custodiabolditalic
> Custodia-BoldItalic.ttf
> custodiaitalic           custodia     custodianormalitalic
> Custodia-NormalItalic.ttf
> custodianormal           custodia     custodianormal
> Custodia-Normal.ttf
> custodianormalitalic     custodia     custodianormalitalic
> Custodia-NormalItalic.ttf
> custodiaregular          custodia     custodianormal
> Custodia-Normal.ttf
> custodiasemibold         custodia     custodiasemibold
> Custodia-SemiBold.ttf
> custodiasemibolditalic   custodia     custodiasemibolditalic
> Custodia-SemiBoldItalic.ttf
>
> I tried a different name, but the result was the same.
>
> On Wed, May 25, 2022 at 7:12 PM Henning Hraban Ramm via ntg-context <
> ntg-context@ntg.nl> wrote:
>
>> Ah, sorry, didn’t know that. I can’t find an error in your code, but I
>> don’t have the (commercial) font to test. What’s the output of the
>> mtxrun font lookup?
>> Did you try a different shortcut from "mi"?
>>
>> Hraban
>>
>> Am 25.05.22 um 18:44 schrieb Stefan Nedeljkovic:
>> > It doesn't work for medium weights, as it is not standard. I took my
>> > solution from Wolfgang, here:
>> > https://tex.stackexchange.com/questions/303648/context-font-weights
>> > <https://tex.stackexchange.com/questions/303648/context-font-weights>
>> >
>> > On Wed, May 25, 2022 at 6:32 PM Henning Hraban Ramm via ntg-context
>> > <ntg-context@ntg.nl <mailto:ntg-context@ntg.nl>> wrote:
>> >
>> >     Hi Stefan,
>> >
>> >     I never used \definebodyfont but either \definefontfamily or
>> >     typescripts
>> >     with \definefontsynonym.
>> >
>> >     Did you try \definefontfamily[custodia][rm][Custodia] ?
>> >
>> >     https://wiki.contextgarden.net/Command/definefontfamily
>> >     <https://wiki.contextgarden.net/Command/definefontfamily>
>> >
>> >     Hraban
>> >
>> >     Am 25.05.22 um 18:20 schrieb Stefan Nedeljkovic via ntg-context:
>> >      >
>> >      > Dear list,
>> >      >
>> >      >
>> >      > I'm trying to set up Custodia in LMTX, but the
>> >     custodiasemibolditalic
>> >      > won't show up. It shows up regularly when I run mtxrun --script
>> >     fonts
>> >      > --list --all --pattern=Custodia
>> >      >
>> >      > I'm attaching the output for reference. Any help would be
>> >     appreciated.
>> >      >
>> >      >
>> >      > Sincerely,
>> >      >
>> >      > Stefan
>> >      >
>> >      > \definefontalternative[md]
>> >      >
>> >      > \definefontalternative[mi]
>> >      >
>> >      >
>> >      > \starttypescript [serif] [custodia]
>> >      >
>> >      > \definefontsynonym[Serif] [name:custodianormal][features=default]
>> >      >
>> >      > \definefontsynonym[SerifItalic]
>> >      > [name:custodianormalitalic][features=default]
>> >      >
>> >      > \definefontsynonym[SerifMedium]
>> >     [name:custodiasemibold][features=default]
>> >      >
>> >      > \definefontsynonym[SerifMediumItalic]
>> >      > [name:custodiasemibolditalic][features=default]
>> >      >
>> >      > \definefontsynonym[SerifBold]
>> [name:custodiabold][features=default]
>> >      >
>> >      > \definefontsynonym[SerifBoldItalic]
>> >      > [name:custodiabolditalic][features=default]
>> >      >
>> >      > \stoptypescript
>> >      >
>> >      >
>> >      > \starttypescript [serif] [custodia]
>> >      >
>> >      > \definebodyfont [default] [rm]
>> >      >
>> >      > [
>> >      >
>> >      > tf=Serif sa 1,
>> >      >
>> >      > it=SerifItalic sa 1,
>> >      >
>> >      > md=SerifMedium sa 1,
>> >      >
>> >      > mi=SerifMediumItalic sa 1,
>> >      >
>> >      > bf=SerifBold sa 1,
>> >      >
>> >      > bi=SerifBoldItalic sa 1]
>> >      >
>> >      > \stoptypescript
>> >      >
>> >      >
>> >      >
>> >      > \definetypeface [custodia] [rm] [serif] [custodia] [default]
>> >      >
>> >      >
>> >      >
>> >      >
>> >      >
>> >      > \usetypescript[custodia][uc]
>> >      >
>> >      > \setupbodyfont[custodia,rm,10pt]
>> >      >
>> >      >
>> >      > \starttext
>> >      >
>> >      >
>> >      > Regular Hamburgerfons,
>> >      >
>> >      > {\it Italic Hamburgerfons},
>> >      >
>> >      > {\md Medium Hamburgerfons},
>> >      >
>> >      > {\mi Medium Italic Hamburgerfons},
>> >      >
>> >      > {\bf Bold Hamburgerfons},
>> >      >
>> >      > {\bi Bold Italic Hamburgerfons},\\
>> >      >
>> >      >
>> >      > \setff{smallcaps}{Small Caps Hamburgerfons},
>> >      >
>> >      > \setff{smallcaps}{\it Small Caps Italic Hamburgerfons},
>> >      >
>> >      > \setff{smallcaps}{\md Small Caps Medium Hamburgerfons},
>> >      >
>> >      > \setff{smallcaps}{\mi Small Caps Medium Italic Hamburgerfons},
>> >      >
>> >      > \setff{smallcaps}{\bf Small Caps Bold Hamburgerfons},
>> >      >
>> >      > \setff{smallcaps}{\bi Small Caps Bold Italic Hamburgerfons}
>> >      >
>> >      >
>> >      > \stoptext
>> >      >
>>
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