A column density is just the integral of a (number) density along the line of sight.
Often, you would get the same observational absorption or emission signature from a number density of n molecules cm−3, spread along a line of sight through a cloud of diameter x, as you would with a density of 0.1n spread over 10x.
In other words, all we can infer is the product or, more accurately, the integral of ndx, and this is known as the column density.
In the paper you refer to, I doubt they know the thickness of the shock they are investigating, so a column density is all that can be determined.
The relative abundance is more straighforward; it is just the ratio of the number densities of the two labelled species.
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