Say you have a part for either acoustic guitar or tenor ukulele, or possibly even electric guitar. What is a convention for indicating these player choices?
I think there are no other conventions than using a clear name like »acoustic guitar«, »tenor ukulele« or »electric guitar«.
You can also get more specific if need be - “Acoustic Guitar (nylon strings)”, “Electric Bass (5 string)”.
Do you mean it could be played on a Guitar or a Ukulele or an electric Guitar?