Semi-formal for men imo means always a jacket, but tie optional. Found this site that has a lot of good suggestions:
https://www.thetrendspotter.net/mens...formal-attire/
For women, something between churchy and formal. The same site has suggestions for women:
https://www.thetrendspotter.net/semi...ss-code-women/
I suppose the trends where the wedding takes place might make a difference, too. Where I live, people tend to be more casual than in a city. There would be some guys in jeans and a polo......