The XMI schema also extends the XML schema so that definitions of objects can be stored. This provives a way to hold a UML model.

用例是什么？有什么作用？

面向对象设计中，如何实现业务逻辑与控制逻辑的分离？

As you note, Activity diagrams inherently can include concurrency and timing. If you look at this example cribbed from Wikipedia, shown below, you can observe the section with two heavy horizontal bars, and two parallel activities of "present idea" and "record idea". That is read as "start these activities in parallel, and continue only when both are complete." Flowcharts can't express this within the notation. Practically, using activity diagrams lets you think clearly about concurrent processes. I think you'll find that anyone who can read a flowchart will quickly adapt.

Activity diagram spreads confusion by its own name, there must be a reason why nobody understand them and ask similar questions.

It might seem as a preference, but if we have a standardized language for describing software systems, Why do we use something else? This can lead to bad habit of overusing flowcharts. Activity diagrams are really simple. But if you decide to describe a more complicated aspect of the system or try to change the part you are describing, you might have to switch anyway. So just use UML and prevent confusion in the future.

Business Process Model and Notation (BPMN) is a standard for business process modeling that provides a graphical notation for specifying business processes in a Business Process Diagram (BPD),[3] based on a flowcharting technique very similar to activity diagrams from Unified Modeling Language (UML).

类之间的关系是我们需要关注的

User stories are short, simple descriptions of a feature told from the perspective of the person who desires the new capability, usually a user or customer of the system. They typically follow a simple template: As a <type of user>, I want <some goal> so that <some reason>. User stories are often written on index cards or sticky notes, stored in a shoe box, and arranged on walls or tables to facilitate planning and discussion. As such, they strongly shift the focus from writing about features to discussing them. In fact, these discussions are more important than whatever text is written.