I want to know the answer to the following question:
Rebirth into the formless realms can only be attained by who?
All the background I'm provided with is that "The above are further subdivided into 31 planes of existence.[web 4] Rebirths in some of the higher heavens, known as the Śuddhāvāsa Worlds or Pure Abodes, can be attained only by skilled Buddhist practitioners known as anāgāmis (non-returners).". Is that enough to answer the question?