They are designated for various reasons, most to accomodate PRMs and Infants:
Example for an A380:
Code:
sm/1/f/v
SM LH 0414/F/02AUGMUCIAD/V
SM LH 0414 F 02AUG MUCIAD 388
A D G K
F 1 UP + A A C UP 1 F
2 UP + A A . UP 2
A D G K
. AVAILABLE <> WING F GEN FACI K GALLEY E EXIT C COT
+ OCCUPIED - LAST OFF H HANDICAP Q QUIET G GROUPS P PET
/ RESTRICTED B BULKHEAD V PREF.SEAT X BLOCKED L LEGROOM U UMNR
() SMOKING D DEPORTEE UP UP-DECK Z NO FILM I INFANT R REAR
Y CHARGEABLE
SM LH 0414/C/02AUGMUCIAD/V
SM LH 0414 C 02AUG MUCIAD 388
A C D G H K
C 10 UP< / A A A A / >UP 10 C
11 < + + A + + + > 11
12 < + + A A + + > 12
14 < + A A A + + > 14
15 < + + A A + + > 15
16 < + + + + A + > 16
18 < + + H A + + > 18
19 < + + H A A . > 19
20 < + A A A U . > 20
21 / A A A A / 21
22 + + A A A + 22
23 + + A A + + 23
24 UP . A A A A . UP 24
A C D G H K
. AVAILABLE <> WING F GEN FACI K GALLEY E EXIT C COT
+ OCCUPIED - LAST OFF H HANDICAP Q QUIET G GROUPS P PET
/ RESTRICTED B BULKHEAD V PREF.SEAT X BLOCKED L LEGROOM U UMNR
() SMOKING D DEPORTEE UP UP-DECK Z NO FILM I INFANT R REAR
Y CHARGEABLE
SM LH 0414/E/02AUGMUCIAD/V
SM LH 0414 E 02AUG MUCIAD 388
A C D E F G H K
E 50 / Y Y / 50 E
51 + + + + 51
52 Y + + + Y Y Y Y 52
53 Y Y Y + + + + + 53
54 Y Y Y Y Y Y Y Y 54
55 Y Y Y Y Y Y Y Y 55
56 Y Y Y Y Y Y Y Y 56
57 + Y Y Y 57
A C D E F G H K
. AVAILABLE <> WING F GEN FACI K GALLEY E EXIT C COT
+ OCCUPIED - LAST OFF H HANDICAP Q QUIET G GROUPS P PET
/ RESTRICTED B BULKHEAD V PREF.SEAT X BLOCKED L LEGROOM U UMNR
() SMOKING D DEPORTEE UP UP-DECK Z NO FILM I INFANT R REAR
Y CHARGEABLE