You need the familyname of the font, you use the font manager of your OS
to get the name.
\definefontfamily [junction] [rm] [Junction]
\definefontfamily [junction] [mm] [Latin Modern Math]
\definefontfamily [junction-light] [rm] [Junction]
[tf=style:light,bf=style:regular]
I just had it made working this way:\definefontfamily[mainface] [ss] [EgalobhierFarinUrlaubodersonstwasoderwersteht] [tf=Junction Light, bf=Junction Regular]
\definefontfamily [mainface] [ss] [Junction] [tf=* Light,bf=* Regular]
It seems that as soon as you define your own weights (which you can mix from different font families) in the fourth pair of brackets, there can be anything in the third pair of brackets (family name).
\definefontfamily[mainface]
[sans]
[Anything]
[regularfont=Junction Light, boldfont=Junction Regular]
works as well and is more self-explaining.
This will only work when you set a upright font or you will get problems.
An advantage of your way
> \definefontfamily [junction-light] [rm] [Junction]
> [tf=style:light,bf=style:regular]
is that you have to type the family name only once if you re-define weights from the same family.
When you use the keywords for the style you don’t have to look for the right names and simpelfonts has fallbacks when the requested style isn’t available.
Are there any reasons why one should not use any of all those synonyms?