So what is exactly the difference in meaning/nuance between these two?

To me it seems you can't swap ulkona for ulkosalla in sentences like this:

>Ulkona oli pari kulunutta pöytää

But in sentences like these you can always swap ulkosalla for ulkona right?

>Vanha mies valmisti kalaa ulkosalla

>Monet seisoskelivat vielä ympäriinsä, tervehtivät toisiaan ja jutustelivat ulkosalla

So what would be the difference if you were to use ulkona there?

For other words in general as well, what would be the difference in using the -sa- form over just a single location case? E.g. kotosalla and taivasalla

Verstas (n.) – Workshop

Example: He rakensivat huonekalut itse omassa verstaassa.

Translation: They built the furniture themselves in their own workshop.

Case	Singular	Plural
Nominative	verstas	verstaat
Accusative (nom.)	verstas	verstaat
Accusative (gen.)	verstaan	verstaat
Genitive	verstaan	verstaiden; verstaitten; verstasten
Partitive	verstasta	verstaita
Inessive	verstaassa	verstaissa
Elative	verstaasta	verstaista
Illative	verstaaseen	verstaisiin; verstaihin
Adessive	verstaalla	verstailla
Ablative	verstaalta	verstailta
Allative	verstaalle	verstaille
Essive	verstaana	verstaina
Translative	verstaaksi	verstaiksi
Abessive	verstaatta	verstaitta
Instructive	—	verstain

You can practice the word of the day by using it in a sentence in the comments below!