This field seems to be an enumeration. Where can I obtain corresponding field values? 840=USD
I should start by saying that for records (RICs) on Refinitiv Elektron datafeed, which provides real-time market data to Eikon, fields that start with "X_" and have negative FID numbers in All Fields view in a Quote app in Eikon such as X_CURRENCY (FID -11) or X_RIC_NAME (FID -1) are service fields that shouldn't be used by anyone. To get the currency of the instrument use CF_CURR field instead of X_CURRENCY.
I cannot help but wonder how you got the enumerated value for this field. Get_data method of Eikon Data APIs cannot retrieve this field at all. And legacy Eikon COM and .NET APIs in my experience return the expanded values for enumerated fields.
Given the above I'm not sure why you'd ever need to examine the enumeration of the fields. But if for whatever reason you do, you can find it in the text file named enumtype.def located in %LOCALAPPDATA%\Thomson Reuters\Cache\<User UUID>\PersistentFiles\Config\RealTime\Display Templates.
Note that X_CURRENCY field is not found in the enumtype.def file, but you can find CURRENCY and CF_CURR fields, which have the same enumeration.