Which SQL table holds the custom user fields on customer records

There is a Dimensions_fields_(version) file which contains all information on the tables and fields in your current version of Dimensions. This file can be accessed in the below location if your Dimensions install is a local one;
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​C:\Program Files (x86)\Access Applications\Dimensions\Accounts\(version)\Extras
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If your Dimensions install is on a server, the location would be \\(server location)\Dimensions\Accounts\(version)\Extras