What do the numbers and letters in the "Intensity" section for strong lines (emission) mean? NIST has a website containing various tabulated spectroscopic data, including the strong lines for various elements. For example, here are the strong lines for Helium. On that page, you can see on the left-hand side a column titled "Intensity", with values for each emission line. My question is simple - what do these numbers mean? How do I read them?
For example, what does "50 c" mean? Or what about "5000 P"?
 A: All of this information is taken directly from the "Basic Atomic Spectroscopic Data Handbook".

The numbers roughly represent relative intensities of the peak of the spectral line derived from the measurements documented in the original source for the data (which is given next to every measurement). I say roughly because there are a few caveats.
Unlike the other tabulated data, the relative intensities are not basic data and must be used with caution. The relative intensities of the spectral lines observed for any element depend upon the light source and excitation conditions. Thus, even if the relative intensities observed in a particular experiment are adjusted to correct for the wavelength dependence of the sensitivity of the spectrometer and detector, the intensities will in general be different from relative intensities given by a previous observer or tabulated in a compilation such as this one. With a caveat that users should keep these considerations in mind, we list a relative intensity for each line. For some lines the wavelengths are so close to another that it was impossible to make two separate intensity measurements. For those lines the intensity of the blended line is given for each and both intensities are marked with an asterisk.
For uniformity we have assigned an intensity of 1000 to the strongest line(s) of each spectrum. In most cases the chosen line (or lines) can reasonably be regarded as the ultimate line (Sec IV). The relative intensities for most spectra here are based on values from [RCWM80]. We have attempted to give improved intensities for some spectra by using more recent and apparently more accurate data than those available to the compilers of [RCWM80].
It should be noted that the intensities in [RCWM80] for lines of neutral and singly-ionized atoms of about half the elements (mainly nd- and nf-shell metals) were taken at least in part from [MCS75]. These intensities were obtained from observations of 10-A, 220-V direct-current arc discharges between copper electrodes having 0.1% of the element under investigation. The relative intensities were put on a linear scale by the use of standardized lamps.
For several spectra we have altered some of the intensities found in the literature to give smoother transitions between wavelength regions covered by different observers. We have also adjusted reported intensities given in a single reference in some cases where the reported values were clearly affected by strong self-absorption and/or by large wavelength-dependent non-linearities. Such adjustments were necessary in order to assign the largest intensities to the inherently strongest persistent lines (usually the ultimate lines).

The letters represent various remarks about a spectral line:

Lines we have selected as persistent (Sec IV) are indicated by the letter "P" following the intensity. For some spectra, other descriptive codes have been included to characterize the line shape or give other related information. They have the following meanings:
b - band head
c - complex
d - line consists of two unresolved lines
h - hazy
l - shaded to longer wavelengths
P - a persistent line
r - easily reversed
s - shaded to shorter wavelengths
u - unresolved shoulder on strong line
w - wide

As for the precise definition of "persistent":

In spectroscopic observations made with low concentrations of a particular element relative to other substances in the source, the number of observable lines of the element is found to decrease with decreasing concentration until only the most "persistent" or "sensitive" lines remain. Some authors refer to the last such line(s) as the raie(s) ultime, i.e., the ultimate line(s). Although the ultimate lines depend in principle on the source, the spectrometer, and other features of the experiment, a relatively small group of lines can be specified for each element that will include the ultimate lines as observed over a broad range of experimental conditions. We designate our selection of these lines "persistent lines."
The strongest persistent lines usually include one or more resonance lines, i.e., transitions to the ground level or term. We include at least one of the resonance lines in our persistent line table for each spectrum. The most sensitive or ultimate lines for many spectra lie in the vacuum-ultraviolet region (wavelength < 2000 Å). In such cases we have tried to include some lines above 2000 Å in the persistent lines list. We have also tried to make these tables more generally useful for many spectra by covering broader wavelength ranges than most tables of this sort. For all transitions with wave numbers greater than 50,000 cm−1 the wavelengths listed are vacuum wavelengths; for those less than 50,000 cm−1 air wavelengths are given.

