inRoute does use a data plan when needed, but maps and routes are automatically saved locally so most functionality remains available without a data connection. Here are some additional details:
The route itself (directions as opposed to map images), your pins and notes, are all stored locally by inRoute, so they can be accessed while offline. During navigation, off-route recalculation isn’t possible without a data connection. In this case, the navigator will speak once that internet/recalculation isn’t available and continue to show your location and the previously calculated route on the map, until you either return to the route or a connection becomes available.
Your device automatically stores Apple’s map data (which inRoute uses) in a temporary cache, depending in part on when it was last used and device free storage. In advance of a trip, you can influence the maps that are stored locally by panning the map through the route at the zoom level needed (e.g. tap “Go” and pan the route). Verifying a map area is cached can be done in Airplane mode. Apple’s map data does not currently support saving a map area to ensure it will always be available when offline. If a cached map is not available during offline navigation, the map will show empty gridlines, although the blue route line will still show and turn instructions will continue.