As the title said, what is a system in mainstream web novels? As I know it system is a group or set of elements that are integrated to achieve desired result or purpose, if you go with the definition then the System is "a set of principles or procedures according to which something is done; an organized framework or method". I have tried to read a couple of system novels but those stories make me a bit confused at first, people get menu screens like in the game, then isn't it a user interface? or entity like AI to guide the heroes, then isn't it just intelligence voice assistant? I had read Mother of Learning and I think Divine Limiter or Structured Spellsasting that mages train is a system where people need to gain and combine to perform magic namely, studies in literacy, languages, and logic since childhood, mastered a few minor but absolutely necessary skills like sense their own mana, quickly and reliably draw upon their personal mana reserves, consciously direct their mana along with specific ways, visualize the desired result with a high degree of detail and clarity and exercise discipline over their thoughts to maintain concentration during spellcasting, also have understanding in magical elements and proxies, memorizing set or hand gestures and chants. For mages to produce a spell they need to learn each knowledge and skill and integrated them, failing in just one element and their spell produce nothing. Also, mages education itself is a school system.
Cheat systems. Basically those game like interfaces. It gives the mc tremendous advantage over other people in his world. It can range from letting you see your stats in the form of points or ranks, give you a mulitversal shop, giving you tasks that has rewards after completion such as currency used for the shop or exp and skills, etc.. Anyways the interface is what the tag is refering to.
It's basically an evolution of the "game elements" tag. In Japanese novels it's still just the ordinary RPG mechanics, but in Chinese novels it's developed into a very different thing. The "system" in Chinese novels is usually a character in its own right, an intelligent being that's treated like a person, can carry on conversations, sometimes has a family, etc., with some control over the actual game systems that lets them bend the rules or let the protagonist negotiate with them over how the mechanics work, so it's less like a game.
When talking about the "System" tag. I think we usually refer to the framework a character's overall power structure is based on, often unique to its owner. It doesn't matter if it can be visualized (UI) or not; as long as it is "something" that lets a character power-up - literally with numerical/level values - and move the story forward, that is a system. The "power system" is another thing though.
Most system novels I read do have a goal. Though methods are often left to the owner of the system, the system still issues the tasks and quests and give rewards upon the completion of those. Not like the owner has a complete control over what you called as user interface, they simply unlock certain privileges after completing tasks. I think it simply is an evolved form of game systems.
Kinda like an AI system that gives directions and prompts and tasks and help to the host? At least in CN QT novels, anyways.