The X-Qlik-ProxyId header contains the ID of the Qlik Sense Proxy Service (QPS) instance itself (that is, the ID of the QPS in the repository database). This value is used by other Qlik Sense services to determine to which QPS that the request originally came in.
The header has the following format:
X-Qlik-ProxyId: ProxyId=<proxyid>; Prefix=<prefix>
ProxyId refers to the actual proxy service and Prefix refers to the virtual proxy prefix that was used when making the request.