There are no NIPs that define this functionality because it kinda is Nostr itself. Implementation details are left for the client developer. nostr:nevent1qqsqtjeh7mrzzxld7jx4tp2pgxls3h6jl7cgejgh3n6m43z36rz0a5qpzpmhxue69uhkztnwdaejumr0dshsz9mhwden5te0vf5hgcm0d9hx2u3wwdhkx6tpdshsz8thwden5te0vfhhxarj9e3xjarrda5kuarcda4k7tnrdakj7e367vj