to consult about, to plan, to report in writing to an equal, a despatch
嗞 centers on formal written communication: consulting, planning, and sending official reports or dispatches between equals.
嗞 combines 口 (mouth) with 兹, which likely provides the sound. The exact historical development is uncertain, but the structure suggests a character related to speech or written reports.
The mouth 口 beside 兹 suggests speaking or communicating in an official manner. Picture a mouth issuing a formal dispatch or consulting on a plan.
For シ, imagine a sheaf of official dispatches: sheaf -> シ, and the mouth 口 is reporting them in writing.