This read-write value selects the sequence-number field length used for multilink PPP fragment reassembly, either short (12 bits) or long (24 bits), and can only be set when the bundle is multilink. An admin picks this to match the peer's MLPPP capability, since mismatched sequence lengths prevent correct fragment reordering across bundled links. For example, setting it to long(2), the default, allows sequence numbers up to roughly 16 million before wrapping, useful on higher-bandwidth multilink bundles.