HMRC requires your income and expenses to be reported under their own set of categories (for example Turnover, Cost of Goods bought, Wages, and so on). To help AccountsPortal build your submissions automatically, you tell us which MTD category each of your accounts belongs to. This is called account mapping.
On the Making Tax Digital page, scroll to "MTD for Income Tax - Account Mapping". You'll see a table split into two parts: Income and Expenses. Each row is one of your accounts, showing its Code, Name and Account Type, with an "MTD Account" dropdown alongside.
For each account, open the "MTD Account" dropdown and choose the HMRC category it belongs to. The categories available match your business type (self-employment, UK property or foreign property). Your changes are saved automatically as soon as you select a value - a "Saved" tick appears next to the account.