"ESLI = Employee Sick Leave Income"

Ding ding ding; good mindreading! That seems likely, since that is a requirement to be stated separately either on the W-2 or as a separate statement. That's for your self-employed client, having a W2 job and self-employment as their Side Gig, because the coronavirus provision for "sick and family leave" for self-employed, on the 2020 tax year filing, needs to take any employer-provided sick pay into the computation for the available amount for that self-employment sick/family leave (if the taxpayer qualifies for this provision at all).