spool used to collect spun thread, generally, wholly
叀 centers on a spool for collecting spun thread, and by extension the idea of gathering things together into a whole or generally.
叀 is a rare character whose historical development is uncertain. It may have originally depicted a spool or a tool for winding thread, but the exact origin is not well established.
The top part looks like a spool with thread wound around it, and the bottom 厶 suggests something private or gathered in. Imagine a spool collecting all the loose thread into one place.
For セン, picture a spool of thread being sent (セント) to a weaver: sent -> セン, and the spool gathers the thread for the journey.