Help please reading through this sparc data and the accompanying scientific paper

im reading through the sparc data and the accompanying scientific paper

SPARC: MASS MODELS FOR 175 DISK GALAXIES WITH
SPITZER PHOTOMETRY AND ACCURATE ROTATION CURVES

right now defining terms is my biggest problem
the reading through i come across the scale length' references are pg page, c colum, l line

pg2 c2 l10
"Column (9) gives the scale length of the stellar disk (Rd)."
what is the scale length

pg2 c2 l13
"Column (11) gives the total H I mass (M _{H I} )."
i take it that the mass in relation to hydrogen . what is the i referencing

pg2 c2 section2.1. line4
i take this to be solar luminosity as a unit
L[3.6]

Scale length refers to the brightness profile when it is modelled as an exponential function (like the paper does). It is defined as the radius at which the central brightness (or whatever else is under consideration) has fallen off by a factor of e.

H I (H-one, not H-i) is a naming convention in astronomy referring to neutral hydrogen atom. Compare II = singly ionised, and III = doubly ionised (edit: obviously not applicable to hydrogen). The 21 cm hydrogen line is produced by H I atoms, letting you map neutral hydrogen distribution and calculate the mass of the (non-ionised) galactic gas.
Wikipedia has an article on this under 'Hydrogen line'.

##L_\odot## is solar luminosity, yes. Same as ##M_\odot## is solar mass.