AltaMail allows you to assign different categories to accounts so that you can group and view different account types together. For example you can group all of your business or personal accounts to view in a single screen. The simplest account category is "Favorites" which can be viewed using the Favorites button in the left side panel.
You can add categories to the left side panel for easy access. View

The "Favorite" accounts are selected using categories. You can choose which accounts appear in the favorites inbox, sent and spam. View

Open the edit account settings screen (View) and tap on 'Appearance'. Tap on 'Category' to select the categories that apply to this account.
You can use the wrench button at the top to delete or add categories. The reserved categories at the top can not be changed.
You can also choose multiple accounts for each category in the app account category screen. Open the app settings (View) - Email Management section and then tap on "Account Categories". You can tap to view all of the accounts in each category. Tap to select or deselect accounts.