A VAST tag may be loaded by a Publisher Ad Server, OVP (online video player), or VPAID/VAST Ad Player.
Typically, if the tag is loaded from an OVP or VPAID/VAST Ad Player, it will be loaded Client-Side. Client-Side means the tag is loaded by the Client (user's device via browser or App).
A Publisher Ad Server may support Client-Side tag loads, but most will load the tag Server-Side. Server-Side means it is loaded by a server (ad server) completely independent of the user's device. Therefore the information about the device (IP address, user agent string, etc) must be passed on the request or else it will be considered invalid.
You should always ask a Publisher how they will load the tag (Client-Side or Server-Side) and send them the corresponding tag format from the LKQD™ UI 'Tag Generator'.
If you have any questions regarding which tag to send to a Publisher (Client-Side or Server-Side), please contact your LKQD account manager.