Accounting for Restricted Stock Units - RSUs

Hello,

I'm valuing Costco (ticker "COST"), and struggling a bit on their accounting in their Consolidated Statement of Equity for their RSUs.

I think I conceptually get how RSUs are accounted for; at grant date basically nothing happens (there is an increase in APIC and an offsetting contra-equity account for the same amount), and then over the service period prior to vesting (let's say 3 years in this case) a stock based compensation amount is recognised (1/3rd of the grant, in each of the 3 years), and the contra-account is reduced by the matching amount each year. The net effect of which is that in each of the 3 years the equity accounts are broadly flat at an aggregate level (each year Retained Earnings is lower than what it would be without the SBC, but at the same time APIC is higher by the same amount each year). Presumably so far that is all correct?

But what is Costco showing in their Consolidated Statment of Equity? They have a line item for "Stock-based compensation", a positive 598 for 2019 in APIC. This I think I'm ok with, it's presumably just the allocation of SBC based on a year of the service period having been "served". But where I get completely confused is there is a line item underneath that for "Release of vested restricted stock units (RSUs), including tax effects", which for 2019 is NEGATIVE 272, also in APIC. Can anyone explain what is going on in that line item? And particularly why it is a negative number?

I've been googling for a few hours now and haven't gotten anywhere!

Many thanks!!

Nick