- all tasks can be performed without a software system; computers are used only to make it easier or faster
- bookkeeping in a small company could be an example
- tasks can theoretically be performed without a software system, but they would require so much time and involve so much risk of human error, that it would be extremely impractical or close to impossible to perform them without using computers
- finding items in an extremely large collection (e.g. a huge library) could be an example
- some banking operations could be even better example
- tasks cannot be performed by a human by their very nature; there must be a software system in place in order to perform them
- controlling an embedded device or processing physical phenomena are examples of this
But we can look at the domains and the categorization from a different angle. Some projects require deep domain knowledge that takes years of hard work to build, others have rather simple, self-explanatory domain, known from everyday life to everybody. Would it sound logical that people working on this latter category tend to focus more on programming skills (languages, patterns, XP practices), whereas those working in a complex domain focus more on the domain itself and as long as the system works, they do not aim to become world class clean coders? It definitely sounds logical to me. It may simply be explained by the amount of effort and energy that is needed to understand the domain, so that no time is left to focus on programming skills. Or there may be other reasons. Regardless of that, one can expect that the simpler the domain is, the more chances of bettering the programming skills will be available.
We can join these two views to say that a programmer is likely to advance their programming skills working on a project that is supposed to help users in something that is generally doable without a computer. Conversly, those working on a system that can do things of which human is incapable, will not be capable to extend their programming skills much.
Where do you want to be and in what order?