It's not a type of user, it's a type of authentication that is the difference between the "ArcGIS Login" and the "Your ArcGIS Organization's URL" methods.
ArcGIS Login method will authenticate agains ArcGIS Online's own directory of users. The directory is a global directory for all ArcGIS Online users across the world. Company accounts will often have their company name appended to the username (eg, firstname.lastname@example.org_companyname) to make sure that the usernames a globally unique).
Your ArcGIS Organization's URL method will work with whatever particular settings have been configured in your organisation's ArcGIS Online account. This will often mean that using your actual company email address without the extra suffix as the username, but more importantly, it allows authentication to be done against your company's own directory service (such as LDAP, Active Directory, SAML, Office365, etc). This means that you can use the same usernames/passwords as every other system in your company. But to do so, you'll need to specify your specific AGOL company URL.