http头中的HG-ACL-ID:1是什么

我正在测试一个应用程序，它包含一个标题HG-ACL-ID，HG-ACL，HG-USER，我不知道这些是什么以及它们的用途。有人能帮我吗

以下是http请求和响应头

GET /nanotech-001/human_grading/api/assessments/4/peerGradingSets/latest HTTP/1.1
Host: 
User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:27.0) Gecko/20100101 Firefox/27.0
Accept: application/json, text/javascript, */*; q=0.01
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
HG-ACL-ID: 1
X-CSRF-Token: LBiyM9kSAA8YHlBLUXQM
X-Requested-With: XMLHttpRequest
Referer: 
Cookie: csrf_token=LBiyM9kSAA8YHlBLUXQM; __204u=4654485718-1388476789706; __204r=; __utma=158142248.881358618.1388476794.1388827712.1388830229.25; __utmc=158142248; __utmz=158142248.1388830229.25.13.utmcsr=class.coursera.org|utmccn=(referral)|utmcmd=referral|utmcct=/ml-004/quiz/attempt; __qca=P0-349638149-1388488065337; user_segment=Prospect; __utmb=158142248.500.10.1388830229; maestro_login_flag=1; CAUTH=Tb7QI7IfVe7siBPLxl1gjyymwc5qZXAKuH8R2DwINdUKnEqDRZn7C8pf2UUbdhmjeQH18RinYNkf5Em6b7ivzg.fAwaIfFYBb4fhYyga1K5Yw.ph7vJk6hG4RnPeLaJZGnfE0x8hdx_Rr0s3g_cvFDCOJl9kQRVpORQqSuq2PXL5nCNksPrU4m0fClcInHonBoi-P7v3JzWku5Y6_pkV4UWsEZKrx309RwtJGwhDUizW0iw2QVrl8aP9Mik-uSdgE948m2UsS4A-BOoUE-MLnel9YoAMSgtekYmtkT1ucWjNqR
Connection: keep-alive

答复：

HTTP/1.1 403 Forbidden
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Content-Type: text/html; charset=utf-8
Date: Sat, 04 Jan 2014 11:22:45 GMT
Expires: Thu, 19 Nov 1981 08:52:00 GMT
HG-ACL: {"submitting":"ri","training":"id","peerGrading":"i","selfGrading":"i","results":"i","phase":"grading"}
HG-User: {"inSignatureTrack":0}
Pragma: no-cache
Server: nginx
Set-Cookie: csrf_token=LBiyM9kSAA8YHlBLUXQM; expires=Mon, 03-Feb-2014 11:22:45 GMT; path=/nanotech-001
Vary: Accept-Encoding
Vary: Accept-Encoding
X-Powered-By: PHP/5.3.10-1ubuntu3.9
Content-Length: 61
Connection: keep-alive

---

请求和响应的类型几乎排除了这一点，但如果我不知道的话，我会说

HG

是对dvcs的提示，

ACL

代表访问控制列表。不过，这在ajax请求的上下文中有何意义仍然是个谜。