Bahrain's official airport rule charges an initial BHD 2 plus a BHD 2 airport-pickup fee, so a 'short' Juffair hop that reads cheap on the meter has already eaten BHD 4. Drivers at the inner curb are legitimate — but compare the Uber/Careem Gate 1 quote before joining the rank queue.
BHD 0.275 fares and the BHD 0.700 daily cap only work with a GO Card, sold at three stations (not, on all reports, at the airport exits themselves), for a BHD 0.500 card fee. At exit 4 without one, you're paying BHD 0.300 cash to the driver — fine, but have the right change.
The airport labels A2 as the fastest route from the airport to Manama (Gateway–City Centre–Seef–souq corridor); A1 threads more inner stops toward Isa Town. Boarding the 'wrong' red bus at zone 4A costs a long tail of traffic-light starts.
Everything now runs through the new Passenger Terminal Building (210,000 sqm, 14-million capacity; operations moved in phases from January 2021) — exit numbers, taxi curbs and the bus zone refer to this building. A PDF or forum post from the old-terminal era will send you hunting for an 'exit' that no longer exists.