The leading letter tells you which object catalog the object belongs to, as follows:
|A||auxcat (Auxiliary catalog)|
|C||csocat (Complementary catalog)|
|D||deepcat (Deep space catalog)|
|F||ftocat (Failed to orbit catalog)|
|L||lcat (low altitude catalog)|
|R||rcat (suborbital catalog)|
|S||stdcat (Standard catalog)|
|T||tmpcat (Temporary catalog)|
Note that there are no prefix letters associated with the ecat (event catalog), or the heliocentric and lunar-planetary registers, since they contain only objects already defined in the other catalogs.
Also, each object with a D (deep space catalog) designation also has an S or A catalog designation which applies when the object is within near-Earth space.
Sequence numbers in the A, D, F, L, R, and S catalogs are assigned sequentially starting with 00001. However, occasional numbers may be missing due to deletions of spurious or reassigned entries.
Sequence numbers in the C and T catalogs are not assigned sequentially.
Sequence numbers in the S catalog are in one-to-one correspondence with the US SATCAT catalog numbers. Thus, S46112 corresponds to SATCAT satellite 46112 (2020-056A).
In exceptional cases, future releases of GCAT may reassign existing catalog numbers. Any such reassignments will be recorded explicitly in an accompanying table. However, users who have found earlier (pre-GCAT) JCAT catalog numbers that crept into my public files have no such guarantee - in partcular, auxcat numbers previously seen in public files may now refer to different objects.
In some cases in the object catalog Parent field an asterisk is appended to the identifier. This is a flag that the launch designation in the Piece field is not what you'd expect from the launch designation of the parent. In the normal situation, for example, S45920 has Piece designation 2020-048A, and its parent S45921 has 2020-048B - both are 2020-048 launch pieces. But S45916 has Piece designation 1998-067RP, and its parent is A09547* with piece designation 2020-011, an apparent contradiction. In fact 2020-011 is the correct launch for both, with launch date 2020 Feb 15, but USSF assigns 1998-067 piece designations and a 1998 launch date for all objects released form the ISS. I added the asterisks to facilitate automatic checking: it lets you know to be careful about the launch date and launch information for this object.