Use this parameter to grant the trusted user Full Access permissions to the mailbox specified by the Identity parameter. ...

Use this parameter to grant the trusted user Full Access  permissions to the mailbox specified by the Identity parameter. This parameter accepts the arguments True or False. These arguments are not case sensitive. When using a CSV file, specify GrantFullAccess True to grant the permission or GrantFullAccess False to not grant the permission. Specifying False will not remove the permission if it is already set. To remove the permission, use Remove-MSOnlineMailPermission.