question: can i assign different sound on zone 1,2,3,4? im using S70XS. (e.g zone 1 is piano, zone 2 is string, zone 3 is brass and so on)

Yes you can.

The ZONE SWITCH can be used on internal data, in one very unique way.

The S70 XS keyboard can transmit on a single MIDI channel when you are in VOICE mode, PERFORMANCE mode, or MULTI Mix mode.
The S70 XS keyboard can transmit on as many as four MIDI channels when you are in MASTER mode.

The S70 XS tone engine can receive on a signle MIDI channel when you are in VOICE mode or PERFORMANCE mode.
The S70 XS tone engine can receive on as many as 16 MIDI Channels when you are in MULTI Mix mode (to accommodate a SEQUENCER)

If you associate the MASTER mode with the Keyboard’s ability to transmit on 4 Channels simultaneously, and the MULTI Mix mode where it can receive on mulitple MIDI channels - you can have a situation where you are addressing four MIDI channels of a MIXING setup at once.

This can be very powerful when it comes to performing on stage.  Just FYI.

PDF: S70XS/S90XS Master mode