It is essentially a TIFF image file with georeferencing information embedded in it (projection, coordinate system, extent, etc.).
GeoTIFF: GeoTIFF is a fairly standard file format which can be read by most GIS software packages including ArcGIS and Google Earth Pro.Two raster-based file types are supported for importing, GeoTIFF and MBTiles. Note that FieldMove does not currently support vector-based file types for import. For the iOS case described here this means using iTunes as the bridge between your computer and iPad. Arguably one of the most important features of FieldMove is the ability to import your own basemaps. This method works great as a "quick-and-dirty" option, requiring little time or setup! I usually make sure caching imagery is the last thing I do before heading to the field and then leave my iPad on Airplane Mode. Note in FieldMove's Settings menu there are relevant options for basemap loading and cache resetting.
Then double-check that when you turn off the WIFI the imagery is still visible (this indicates it is temporarily stored, and will remain stored until you clear the cache or overwrite it by loading other areas after). Repeat this procedure with the other imagery types for the same area if desired. Repeat this until you have covered the entire field area. Once the imagery for this area is loaded (clearly visible), move to the adjacent area and let that load. To cache imagery you should zoom on the map to the maximum imagery quality that you desire. Google Earth and most web browsers operate similarly. Think of the cache as a rolling temporary memory- once the cache limit is reached the most recently viewed imagery overwrites the older imagery. If these three layer options are sufficient for your mapping needs and the field area is not overly large, you can temporarily "cache" the imagery to the iPad's internal memory.