blackhole defaults

I don't think I've specifically set any of these:

$ sysctl net.inet | grep blackhole
net.inet.tcp.v6pmtud_blackhole_mss: 1220
net.inet.tcp.pmtud_blackhole_mss: 1200
net.inet.tcp.pmtud_blackhole_detection: 0
net.inet.tcp.blackhole: 2
net.inet.udp.blackhole: 1
net.inet.sctp.blackhole: 0

Can someone explain to me the logic behind the last 3 (tcp,udp,sctp)
defaults, and why they aren't all at 2?

Gary