Q:

Are terminology & comparison related questions on-topic? 

As an example, difference between project manager and business analyst is related to two terminologies and asks about their difference.  Are such questions considered on-topic on PMSE? 
I left a comment for OP to add some details but the response from another user gave an impression that the original post is fine. 
I am basing my opinion on the following:

Have you thoroughly searched for an answer before asking your question?  Sharing your research helps everyone.  Tell us what you found and why it didn’t meet your needs.  This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers.  
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You should only ask practical, answerable questions based on actual problems that you face.  
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This site is NOT about...
Recommendations for blogs, books, links, or general terminology. 
  https://pm.stackexchange.com/help/on-topic

A:

TL;DR
Most of these questions will be off-topic.  There may be occasional exceptions, but I wouldn't hold my breath. 
The question cited was answerable, but I personally consider it a poor-quality question.  It could have easily led down a rabbit hole, but in this case did not.  I'm inclined to leave it open, but would certainly revisit this particular question if it starts to attract a lot of context-dependent answers in the future.  The community's mileage may vary, and I certainly wouldn't object if the question was closed as "too broad" or "hard to tell what you're asking" because of its likelihood of drawing anecdotal or polling-response answers. 
Further Analysis

As an example, difference between project manager and business analyst is related to two terminologies and asks about their difference.  Are such questions considered on-topic on PMSE? 

Maybe.  Sometimes.  Occasionally. 
"Why is a raven like a writing desk?"  is certainly off-topic.  On the other hand, questions about roles within a project might be relevant. 
The problem with many of these questions is that the answer depends on context not provided.  There is often no canonical answer, so it becomes a question about what a given role or title means at a particular company or for a given project.  However, most of these sorts of questions are looking for a less contextual, more absolute answer. 
In addition, most of these questions are either searchable (e.g. "What are the roles in Scrum?")  or hard to answer in the abstract (i.e. "Does a project manager have any business analysis responsibilities?").  So again, a question without well-defined context, a clear problem statement, or exacting precision will either be too broad or too opinionated. 
In the question cited, other than saying they're usually different roles with different responsibilities, there's no context about why the OP considers this a difficult distinction.  Is a business analyst responsible for project planning and management?  Usually not, but we have no idea how the project or company is structured.  Is a project manager ever responsible for requirements analysis and documenting specifications?  Maybe, sometimes, and occasionally, especially depending on the framework and if we stretch "project management" to include agile-ish or hybrid roles. 
To sum up, it's a low-quality question, but I currently consider it on-topic.  Because of its subjectivity, other opinions on its topicality are likely valid, which is the point of why this sort of question is problematic. 

A:

I see this in a slightly different approach. 

What's more important?  To accept as valid some questions that cannot
  have a canonical answer or to leave the community without any answers
  at all? 

I'd support the former. 
My reading of "off-topic" for general terminology was to avoid questions that are broadly answered all over the internet: "what's agile?" " what's scrum?".  That's my reading. :) 
The specific mentioned question has an underlying question that many other fellow PMs might have (heck, too many titles and I still don't know who should do what in my project!).  The answer cannot be canonical (context dependent, as CG mentions) and because of it, could tagged as off-topic.  
On the other hand, being (I'd bet) a fairly common question (like many other terminology questions), I'd consider it valid - it'll be useful for other (potential new) users... even if the answer is "go ask in your company, it fully depends on it".  If we leave these questions open, we'd be leaving a gap that could not be answered anywhere else at SE community. 