Spare parts moved from one line's machine to another instead of being purchased new. Cost lands on the receiving line; the donor line is flagged here until a replacement is purchased.
To record a new transfer: open the receiving line's drawing, click the station, add the part, and set its Source dropdown to the line it came from.
To move an entire machine (not just a part) to another line, use the button next to a station in that line's Process Sequence table.