Q:

Asking questions with UML diagrams instead of code

Can I ask a question with a UML class diagram instead of code? What about instead of some code? The kind of questions I'm interested in asking are about software architecture, not low-level coding issues.

A:

If the entire question consists of UML diagrams and no code, then it's off-topic for this site.  Overall, the specific code to be reviewed must be embedded in the post itself.  It's okay to include UML diagrams as supplemental material, but we cannot review that specifically.  If your question is primarily about UML diagrams (software architecture as you say), then it won't fit on this site.
Here's the relevant off-topic section found in the Help Center:

However, if your question is not about a particular piece of code and
  instead is a generally applicable question about…

Best practices in general (that is, it's okay to ask "Does this code follow common best practices?", but not "What is the best practice
  regarding X?")
Tools, improving, or conducting code reviews
How to add a feature to or solve a problem with your code
Trouble-shooting, debugging, or understanding code snippets
Higher-level architecture and design of software systems

then your question is off-topic for this site.

