What is the voluntary band plan restriction for US stations transmitting within the 48 contiguous states in the 50.1 MHz to 50.125 MHz band segment?
Why The 25 kHz from 50.100 to 50.125 MHz is the 6 meter DX window under the voluntary band plan. Because 6 meter openings to other continents are rare and weak, stations in the lower 48 states keep that slice clear for contacts with stations outside the contiguous 48 (DX, Alaska, Hawaii, Caribbean, etc.). Domestic stateside SSB work starts at the calling frequency 50.125 MHz and above. This is a band plan convention, not an FCC rule, but it is widely honored.
Watch out The choice limiting the segment to contacts within the 48 states is exactly backwards; that is what the rest of the SSB portion above 50.125 MHz is for. Digital and SSTV activity has its own segments higher in the band.
50.100-50.125 = the DX window: if the other station is inside the lower 48, move up to 50.125+.