What does a skill's "rate" field do?

[Lord Estraven]

From s_info.txt

# N:idx:name
# D:desc
# A:action mkey:action desc
# I:rate

What does the bolded field do? I looked through the relevant code sections, and still have no idea what it does. Changing it doesn't seem to change the efficacy of a skill, or the rate at which it is gained, or... anything, really.

Is this field obsolete, or something?

[Yottle]

It is at 1000 for all skills except the Sneakiness ones and some of the Monster-Lore ones, where it is 0. I can't think of anything unusual about those.