While looking at the data Apple sends back and forth for the Safari Surf Suggestion feature I came across the HTTP header “X-Apple-Subscriptions”.
It seems to contain a list of services you have subscribed to using iTunes.
Interestingly, “X-Apple-Subscriptions” yields no search results on Apple Developer Forums, Google, SO, reddit et al.
Can anyone else provide other examples for a) data send using this header b) non-obvious situations in which it is sent?
Request
Host  | api.smoot.apple.com  | 
Accept  | */*  | 
Proxy-Connection  | keep-alive  | 
X-Apple-CachedResults  | %5B%5D  | 
Accept-Language  | en-us  | 
Accept-Encoding  | gzip, deflate  | 
X-Apple-Subscriptions  | [{"type": "premium","provider": "www.netflix.com","active": true},{"active": true,"attribute": "premium","provider": "fuse"}] | 
X-Apple-CachedQueries  | %5B%22bec%22,%22be%22%5D  | 
X-Apple-UserGuid  | cb9c7a5a-d9f04ab1e5a1-f44a7241a4ca  | 
User-Agent  | SafariShared/11601.4.2 (Mac OS X 15D21) Version/9.0.3 Safari/601.4.4  | 
X-Apple-FirstPartySessions  | {"temp_maps_session" : "0037032267291264137303408989036777965119"} | 
Connection  | keep-alive  | 
X-Apple-UI-Scale  | 1.0  | 
X-Apple-Languages  | ["en-US","de","en"]  | 
Pragma  | no-cache  | 
Cache-Control  | no-cache  | 
GET-Parameter
q  | beck  | 
locale  | en-DE  | 
key  | granite0323  | 
latlng  | 52.485000,13.439500  | 
geosrc  | wifi,73.993243  | 
storefront  | 143443-2,32  | 
time_zone  | Europe/Berlin  | 
24h  | true  | 
calendar  | gregorian  | 
region  | en-DE  | 
keyboard  | de  | 
units  | SI  | 
Response
HTTP/1.1 200 OK  | |
Access-Control-Allow-Origin  | *  | 
Cache-Control  | private, max-age=300  | 
Content-Encoding  | deflate  | 
Content-Type  | application/json; charset=utf-8  | 
Date  | Wed, 27 Jan 2016 14:59:33 GMT  | 
Content-Length  | 1446  | 
Strict-Transport-Security  | max-age=15768000  | 
Expires  | 0  | 
Cache-Control  | no-cache  | 
Payload siehe http://pastebin.com/AXweszJn.