The tax domicile determines in which country you/your professional must pay tax on the income. Legally, this should be the address where you/your professional actually live most of the year. The rule of thumb that is often used here is to literally look at the country where you/your professional reside for more than half of the year, which is 183 days.
We request the tax domicile to find out what documents are needed for the preparation of the agreement (e.g. A1 form, residence permit, permits etc.).