Preferred transfers make exceptions to the general rules of the Google Maps (and other online mapping applications), helping them understand your GTFS data differently when special situations arise. One of the most important rules is that in providing directions that require transfers between routes, Google Maps assumes that it will take walking time between the stops plus 3 minutes as the required time to make a transfer happen. That means that even if a transfer between two routes happens at the same stop, Google maps will only suggest the transfer if the scheduled departure times are at least 3 minutes apart.
There are different types of preferred transfers.
Recommended transfer: This does not change required transfer interval values. If Google Maps sees a few transfer opportunities (i.e. two routes intersect at multiple points), and various itineraries that use these transfer points are equal in terms of overall travel time, then Google Maps will show the the "recommended transfer".
Timed transfer: This is a timed transfer point between two routes. The departing vehicle is expected to wait for the arriving one, with sufficient time for a passenger to transfer between routes. This label thus tells Google Maps to override the 3 minute assumption.
Requires minimum time: This transfer requires a minimum amount of time between arrival and departure to ensure a connection. The time required to transfer is specified by minimum transfer time.
Transfers not possible: Transfers are not possible between routes at between these locations.
**Note that you cannot have the same ordered stop pair entered as a timed transfer and also as a recommended transfer. An error will be returned. Timed transfers are automatically treated as recommended transfers.