In civilian applications, it is not legal to lock doors to
prevent people from leaving the building since that affects life safety.
So the only lockdown you can do as a security system installer is to
lock doors so people cannot get in. Local authorities having
jurisdiction (LAHJ) in some cases can decide to allow a delayed exit
from buildings and some goverment agencies do not have to follow the
rules of LAHJs so they can stop exit from a building.
One of the fastest growing use for lock downs is for schools and
universities. Well publicized events have made this area highly visible.
Lock manufacturers have come up with many solutions to enabling remote
locking to prevent access to key areas.
Complete PC-based lock systems are available from several
manufacturers as discussed in the complete article at the link below.