VMS Help Sys Parameters, TTY_ALTALARM  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    TTY_ALTALARM sets the size of the alternate type-ahead buffer
    alarm. This value indicates at what point an XOFF should be sent
    to terminals that use the alternate type-ahead buffers with the
    size specified by the TTY_ALTYPAHD parameter.