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]

Sorry, my question was very abstractive. I add some pictures with green arrow. What does it mean? Unlike 'x' in the picture you add, the lines in the picture I add are linked.

[bjorn-spockeli]

Which schematic is this? It looks like it is from one of our nRF51 or nRF52 DKs?

[J.Kwon]

Yes, it is the schematics of PCA10028.

[bjorn-spockeli]

The meaning is the same, i.e. it's not connected to VDD.

[J.Kwon]

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