This read-write string sets the Maximum Received Reconstructed Unit, the largest number of octets allowed in the information field of a packet after multilink PPP fragments have been reassembled, and can only be changed when the bundle is configured as multilink. It matters to an admin sizing multilink bundles because it caps how big a reassembled packet can be before the far end must refragment it across links. As an example, the default "1500-1524-8192" lets the bundle negotiate a reconstructed-packet ceiling up to 8192 bytes.