To answer your question directly, 'ud' stands for 'user data'.  You will find a description of these fields in the RestAPI user guide available at [URL]  You should be able to find similar RestAPI guide for other XtremIO versions at [URL]  Hope this helps.
Connecting to XtremIO clusters via Rest over https and receiving the JSON content back successfully. I will be creating histograms via RRDTool for IOPS, MBs, and Latency relative to block sizes as well as collecting and tracking rates and capacities of dedup, thin/oversubscription, physical space, physical used, logical used, allocated, etc. I am unsure of what some of the terms mean relative to the other data. An example of a Rest query on '/api/json/types/clusters/1' produces a content listing with some of the following:  HTTP/1.1 200 OKDate: Tue, 03 Feb 2015 14:29:34 GMTServer: TwistedWeb/8.2.0Content-Type: application/jsonStrict-Transport-Security: max-age=16070400;includeSubdomainsTransfer-Encoding: chunked {     "content": {         "wr-bw-64kb": "8998", ..        "ud-ssd-space-in-use": "2579321840", ..        "space-in-use": "2579321840", ..        "ud-ssd-space":  "16052147968", ..     }}  Just wondering things like - What does 'ud' stand for in the description of some of the content terms? So I was wondering if there is a dictionary relative to the data available from the JSON output of an XtremIO array? Are there different dictionaries relative to Code level running in the cluster?   Did not know if any of this information is available to customers?  
XtermIO - Rest API/JSON content Structure - Are there any Guides/Dictionaries?