The uncertainty codes used by the HITRAN database are described in Table 5 of the HITRAN2004 paper [1], which is reproduced here. There are two types of uncertainty code corresponding to absolute uncertainty in cm-1 (used for the line position and air pressure-induced line shift parameters) and relative uncertainty in % (used for the line intensity and broadening parameters).

CodeAbsolute Uncertainty range CodeRelative Uncertainty range
0≥ 1 or Unreported 0Unreported or unavailable
1≥ 0.1 and < 1 1Default or constant
2≥ 0.01 and < 0.1 2Average or estimate
3≥ 0.001 and < 0.01 3≥ 20 %
4≥ 0.0001 and < 0.001 4≥ 10 % and < 20 %
5≥ 0.00001 and < 0.0001 5≥ 5 % and < 10 %
6≥ 0.000001 and < 0.00001 6≥ 2 % and < 5 %
7≥ 0.0000001 and < 0.000001 7≥ 1 % and < 2 %
8≥ 0.00000001 and < 0.0000001 8< 1 %
9≥ 0.000000001 and < 0.00000001

References

[1] L. S. Rothman, et al., "The HITRAN 2004 molecular spectroscopic database", J. Quant. Spectrosc. Radiat. Transfer 96, 139-204 (2005). [link to article] [ADS]