boundary, limit
埏 centers on a physical or conceptual boundary: a limit, edge, or demarcation, often related to earth or ground.
埏 combines 土 (earth) with 延, which likely contributes the sound and the idea of extending or stretching, suggesting an earthen boundary or limit.
The left side is earth 土, and the right side 延 suggests extending. Picture a long earthen wall stretching out to mark the boundary of a field.
For セン, imagine a surveyor using a cent (セント) coin to mark the exact boundary line on the ground: cent -> セン.