Some linguists restrict the double-dot subscript ⟨◌̤⟩ to murmured sonorants, such as vowels and nasals, which are murmured throughout their duration, and use the superscript hook-aitch ⟨◌ʱ⟩ for the breathy-voiced release of obstruents.

Answer this question, if possible (if impossible, reply "unanswerable"): All linguists restrict the double-dot subscript to what?
unanswerable