Ask specific question to a particular sect? E.g. asking question to Shias only or asking question to Sunnis only etc.
Is this allowed or not? And if not then why?
As the site itself is non-denominational, questions pertaining to any and all flavors of Islam are welcome. As long as they're related to the topic of Islam, questions soliciting answers from a specific perspective (e.g., Shi'a-only, Hanafi-only, Qur'an-only, etc.) are just as welcome as those which are scoped more generally. Merely specify in the body of the question which perspective you're seeking an answer from; any answers from an irrelevant perspective can thus be flagged and/or deleted.
Tagging questions by denomination may also be appropriate, however it is unwise to rely on tags alone for specification; as tags can be easily overlooked, the perspective required should still be mentioned in the question body itself. Otherwise it's likely to solicit answers from anyone.
If the phenomenon in question is obviously unique to a particular sect then a simple tag is more than enough.
We don't explicitly state it's a Python question on SO when it is evident from the code and experts (same thing which we want on this site) quickly check the tags if it's not clear, just to make sure. Plus tags in most cases are the determining factor (e.g. https://stackoverflow.com/q/30081275/1309352)
E.g. in the question Is it true that all the infallibles are destined to receive martyrdom? the phenomenon under question is unique to the shiite school of belief therefore it does not need to be any more specific within the question body. We are better off not getting answer from a user who actually is not knowledgable enough to discern which sect the phenomenon under question belongs to or read the quesion carefully.
Places where it is advisable to state in the question body the denomination and perspective from which an answer is sought is e.g. What is the ruling of shaking-hands with a non-mahram who is non-Muslim?. This question could get different answers based on the perspective so better state at then have the community do the cleaning.'
Evidence abound, there are plenty of instance when the perspective sought is clearly stated in the question and tag yet it recieves answers from another perspective.