sorry I was vague!

The ischial tuberosities are about 1-1.5 cm wide. The outside measurement is from the outside of one to the outside of the other. That span helps you figure out how wide the overall saddle must be (add some wiggle room for shifting around for cornering and such, like 1-2cm each side)

Center to center is the weightbearing part of the tuberosity, like the contact point, where you put the most pressure on the saddle. This is what you get if you sit on a butt-o-meter. Your c-to-c should match the saddle cheeks c-to-c, your contact points should match the engineered contact points of the saddle. (in the center of the domed portions of a Serfas Niva, for example, rather than missing the dome and hitting on the slope, thereby causing the padded dome to mash up into the soft tissues) C-to-C may only matter for saddles with topographically interesting top surfaces, not so much for flat or unpadded top surfaces where there isn't doming and grooving and cut-outs.