what is the red 'x' in the schematic?

[J.Kwon]

I don't think it means cut..

[bjorn-spockeli]

If you're referring to the red x markings in our Reference Circuitry Schematics, see below, then as @RK correctly points out they mean N.C , i.e. No Connection.

[J.Kwon]

If so, does not the exteranl crystal oscillator(X1) in the pca10028 work?