I'm starting to learn how to use map packages and tile packages for Collector. If I create a custom offline basemap in a state plane projection for use in a tablet how are new features stored? If I use the tablet GPS location to add a point feature is the new feature added in state plane?
You will need both the feature class and the basemap to be in the projection you are wanting to use. This means creating a custom basemap and uploading it to AGOL and then finally downloading it onto your tablet. This is the only way to use custom basemaps with a custom projection. Sideloading is not an option due to the way the projection data is generated and stored.