It is just showing (awkwardly, I admit) that the result of this calculation is the value for line 3, and not the 'total days as Massachusetts resident'. You can see this elsewhere on the form, e.g. line 27:
I did call FTB to confirm. The mailing address to contact them is the one listed on the upper left-hand corner of the notice itself. Reporting back so that others will know where to go when faced with a similar situation.
Thanks all for your replies.
Illinois counties collect property tax and sales tax, but not income tax. (Source: long-time Illinois resident.)
Also see, for example, this proposal to start having a Chicago income tax here, which notes that:
The City does not impose an income tax on residents or workers in Chicago. All residents of Illinois, including Chicago ...
I focused on the state government:State and Local Taxes Imposed :
Below are the State and Local Taxes that have fixed rates.
Adult Use Cannabis Cultivation Privilege Tax
Hotel Operator’s Tax
Individual Income Tax
Liquor Gallonage Tax
Withholding (Payroll) Tax
Illinois Sports Facilities Hotel Tax
Metropolitan Pier and ...
A negative is harder to prove than a positive, but this page suggests that Illinois is not one of the state's that has a county-level income tax.
If you need this information for decision-making purposes, I would recommend contacting the county in question directly.