Domicile: your true home
Your domicile is the one place you treat as your permanent home and intend to return to. You have exactly one at a time, and you keep it until you clearly establish a new one. States look at your home, where your family lives, where you work and do business, where you keep things that matter to you, and where you spend your time.
Statutory residency: days plus a home
Many states also tax people who aren't domiciled there but spend enough days in the state. In states like New York, Connecticut and New Jersey, that means more than 183 days plus a permanent place to live in the state. See every state's test.
Why both matter
You can change your domicile to Florida and still be a statutory resident of New York for a year if you keep an apartment there and spend more than 183 days in the state. And staying under 183 days doesn't help if the state decides you never really changed your domicile.
Where days fit
Days are a bright line for statutory residency and also strong evidence for domicile: where you spend most of your time says a lot about where home is. A complete log helps with both.