The "User Account" are created under the "Users" section to allow other users to access Plesk in order to manage websites, install applications, or use email services under your domains.
When "User Account" is created under "Users" tab, the account creation wizard will provide you with options to create an email address under your account or to use an external email address. If you select the option "Create an email address under your account", then an email account will be created and will be shown under "Mail" tab.
Under "Mail" tab, you can create and manage email accounts associated with your domains. When you create and email address from "Mail" section, the "Create Email Address" wizard will provide you an option to provide access to the Control Panel. If you do not provide access to the Control Panel for the email address, then this email address will not appear under "User Account" tab. If you provide access to control panel for the email address you created, then this email address will appear under "User Account" tab.
If you wish to verify this by yourself, please follow the below steps:
1) Login to your Plesk control panel
2) Click on "Subscriptions". Select the desired subscription.
3) Click on "Email Addresses" tab under "Mail" section.
4) Click "Create Email Address"
5) Enter desired values in the "Email Address" and "Password" fields.
5) Tick or untick the "Access to the Control Panel" option.
6) Click "OK" to save the settings.
If you did not tick the "Access to the Control Panel" option, then the new email address you created will only appear under "Mail" tab. This email address will not be shown under "Users" tab.