When do I need to file REG-1?
When people start a business, they generally assume they need to register the business with the state. While business entities, like corporations and LLCs are created by registerin with the Secretary of State, actual business registration is done with IDOR via REG-1, the Illinois Business Registration Application. This also lets you create a MyTax Illinois account. However, REG-1 is very focused on reporting what taxes you owe, so if you have a business that does not owe any new state taxes, it seems inapplicable. To confirm this is the case, I asked IDOR, and they promptly replied with the following:
The function of the REG-1 is to register a business for tax accounts in which they have a tax liability.
If you are a Single Member LLC or Sole proprietorship you would make your business income tax estimated payments to your 1040. If you are any other organization type you would want to fill out the REG-1 to get the Business Income Tax account for estimated payments.
So, if you have to remit epmloyer income tax withholding, unemployment, sales, or excise tax, you have to file REG-1, file those returns, and make those payments. If don't have employees and don't make retail sales, you probably don't have to. Do be warned that IDES's definition of who is an employee versus a contractor is different than the IRS's. Also, even if you don't have to register to remit sales tax (say because you sell wholesale or through a marketplace), you make still want to register anyway so you can get an IDOR account ID, so you can fill out a CRT-61, Certificate of Resale, for your own purchases.