What type of business was it? What kind of functionality did this TUI application provide?

It was a customer management and billing system, for a specific industry. Tailored to local requirements and insurance regulations (a lot of things got billed to insurance). There is commercial software available for this purpose, but it's a very small target group, and the products are neither great, nor cheap.

Over time it even got extended to interface with a few external systems, mostly by reading and writing text files to specific locations.

It was deployed on one location with 2-5 concurrent users on the same database. The workflow was something like this: open customer/new customer, read/add notes, register products/services provided, print invoice for customer, print monthly invoice for insurance companies including all the required information (hundreds of pages).