Absolutely (Almost) Everything
Specify is one part of the two part stack of Specify and Specified.
Specify collects data from the system it's running on and then passes all this data to a website; spec-ify.com, henceforth Specified.
Specify probes various WMI and win32 classes on the host machine for an exhaustive list of data, parsing and pushing keyed information into a JSON standard object tree.
Specify is developed with the intent of being used as a quick way to relay the state of one's system to another person, as such, the data gathered is always whatever is relevant for the diagnosis of a tech issue.
There is no explicitly sensitive or private information that gets passed in. This does not necessarily mean that there is no subjectively sensitive or private information that does get included into the snapshot that Specify generates.
There is a good-will effort to give the user the option to omit certain pieces of information that can be most sensitive, the username of the host machine, and the path for commercial OneDrive instances, which by nature include places of work or education in their path.
The snapshot generated by Specify is as previously mentioned, a JSON object tree that is passed to Specified via an internal POST request. This snapshot lives as a .json file in the webserver's file architecture so Specified can ingest and format the information into the desired view.
This JSON file has an expiration date of 24 hours, after which it is deleted automatically. This does mean that your snapshot, should you want to revisit it, will potentially have to be re-ran.
If prompted, Specify will upload your recent Minidumps to our server, for others to analyze. These are also automatically deleted after 24 hours.
No information is otherwise kept in storage on the server, and no cookies are set by Specified.
As previously mentioned, Specify automatically POSTs the results of its runtime to Specified and then automatically opens the customized page, while also copying that URL to the clipboard.
Alternatively, whenever Specify is unable to complete said POST request, or is set to not upload by the user, the .json file is instead written into the same directory as the Specify executable is ran from. This .json can then be manually uploaded to Specified to generate the view.