large jade tablet used by official
玠 refers to a large jade tablet used by officials, likely as a ceremonial or authoritative object. Its meaning is narrow and specific to this artifact.
玠 combines 王 (jade) with 介, which likely contributes the sound. The character is rare and its exact historical development is uncertain.
The left side 王 is jade, and the right side 介 looks like a person standing between boundaries. Imagine an official holding a large jade tablet between his hands as a symbol of authority.
For カイ, picture a kite (カイト) shaped like a large jade tablet flying high: kite -> カイ, and the official's tablet is as grand as a kite in the sky.