A is for an entry that has been identified as a probable astrometric binary by 
A  MK; this presumably has a long period, and so if the system is already known 
A  to have a short period (from SB9, GCVS, or another source), we assume the 
A  system is triple (at least). If this makes it a triple that is not in the 
A  MSC, we write  n= 3?  rather than  n= 3 . Quite often it is in the MSC, 
A  however, because there is convincing additional evidence.
b is for an entry of two or more stars that are (usually) intrinsically bright, 
b  distant and not very close together. They are close enough together that 
b  their juxtaposition is unlikely to be chance, yet far enough apart that one 
b  can question whether they are in a long-lived orbit. It ends up as largely a 
b  subjective matter whether one thinks these should be listed as one system, or
b  two or more systems. In some cases, they may be members of a cluster, and 
b  (arguably) not much closer together than cluster stars usually are. We list 
b  eight systems that we qualify with b.
C is for an entry that comes from CHARA.
E is for an entry identified in the GCVS as having an eclipsing, or eclipse 
E  related, light curve. If a system merits both A and E we regard it as a 
E  probable triple, since an orbit that gives eclipses will usually be too small
E  to also give an astrometric indication. Most but not all systems flagged E 
E  from the GCVS have known periods, coinciding with SB9 periods, but those 
E  without periods are presumably rather uncertain.
G is for a system whose parameters are taken from the catalogue of 
G  R.E.M. Griffin (to be published). These are mostly G/K giants plus B/A 
G  dwarfs; but in a handful of cases, she concludes that the dwarf is itself in 
G  a subbinary.
H is for a system that we identify, through slightly discrepant proper motions, 
H  as having an extra, unseen, component making the system at least triple. 
H  There are six such systems; we assign their multiplicities as 3?, indicating 
H  that we think that the probability of the extra body is over 50 per cent, 
H  though nothing like certain enough for inclusion in the MSC.
h is for a binary system where we suspect that large but uncertain discrepancies
h  in the HIP values of parallax and/or proper motion may be hinting very weakly
h  at a third body. We assign multiplicity 2?, and we identify 25 such systems.
L is for a system where we have relied on the discussion by Lindroos (1985, 
L  Cat. <II/127>) as to whether certain companions, usually rather distant, are 
L  optical or physical, and arguably pre-main sequence if physical.
P is for a system noted by Parsons (2004, Cat. <J/AJ/127/2915>) as containing a 
R  hot component that may be two roughly equal components.
R is for an entry identified as a probable or certain spectroscopic binary by 
R  Famaey et al. (2005, Cat. <J/A+A/430/165>), Andersen & Nordstroem 
R  (1983A&AS...52..471A, 1983A&AS...52..479A, Cat. <III/127>), Nordstrom & 
R  Andersen (1985A&AS...61...53N, Cat. <III/127>) or Grenier et al. (1999,
R  Cat. <J/A+AS/137/451>, Cat. <J/A+AS/135/503>).
s is for a specific reference which is given in the electronic version of the 
s  table.