This is one of those distinctions that is easy for us to take for granted because we work with VTScada every day. It came up a few times at the ACE 2017 conference in Philadelphia.
VTScada Thick Client Licenses are each installed on individual Windows computers. Every VTScada application needs at least one installed Thick Client license. A separate license is required for each PC that has VTScada installed. The price of the license depends on the number of I/O in your system, what optional features are enabled, and whether it is a Runtime (workstation only) or Runtime/Development (workstation and application configuration). Also, any networked Thick Client computer can be a redundant server with automatic failover and data synchronization.
More Information: https://www.trihedral.com/software-licensing
VTScada Thin Client Licenses provide operator access to the application from computers or devices that do NOT have VTScada installed on them. Thin Clients are licensed per concurrent connection from PCs, Macs, smartphones or tablets. Thin Clients are not tied to specific devices. Authorized users use a web link to open a session in a HTML5-compliant browser. Depending on your screen size and data plan, you can choose a simple tag-and-trend view or full display view identical to an operator workstation. Thin Client access requires at least one VTScada Thick Client license with one or more Thin Clients enabled. VTScada does not require any third-party web server software. Thin Clients cannot act as redundant application servers.
More information: https://www.trihedral.com/scada-thin-clients