What is a front sector?

Im not really sure what a front sector is, can someone explain or elaborate for me and tell me how to create one?

(jeez ive been posting a lot lately... ok this will be my last topic for asking questions.)

Take a look at this pic from SRB2 Doombuilder:

The sector in front of the side of the red linedef that has a line sticking out of it is the 'front sector' whilst the other sector would therefore be the 'back sector'. Those names are used for when 2 sectors have to be used for certain linedef actions to work.