There is a difference in how subaccounts, workspaces and users function, and also how these should be used.
Subaccounts are accounts that are managed by a main account. Subaccounts have almost all the functionality that the main account does and have their own login, but it does not have control over its own balance. The main account holder is responsible for making sure the subaccount has enough balance, and can choose to share their own balance with the subaccount or appoint a certain amount of balance to the subaccount. You can see statistics for each subaccount separately. Subaccount are also not able to make or receive Voice Calls.
Subaccounts are useful if for instance you have several branches or your company and want to give them their own login and keep their messaging separate, but you all billing should to go the same department.
Workspaces are separate accounts, but with the same email address. By going to your workspaces overview, you can switch between your workspaces quickly. Billing and statistics are separate for each workspace.
Workspaces are useful if you have two separate businesses that you want to send SMS for, but these are not related and billing is handled by separate entities.
Per account you can have multiple users. Each user will log in to the account with their own login credentials, but they will all see the same dashboard. We do not differentiate between users in statistics so you will not be able to see what was sent out per user.
Different users are useful if you have one company with multiple departments who are looking to send SMS, but you do not need to make any differentiation in billing or statistics.