The discussion in the Russian Wikipedia is one of the users who are not very familiar with Wikidata, arguing that it's not possible to enter a URL as a source, only element. As it turned out, he used to enter the field with the wrong data type. Most likely, this is not an isolated case, and need a way to indicate the type of data (or data input format) for field with which the user is working.
I suggest using the same method as that already used in the VisualEditor, as it is familiar to users. At the beginning of each field to add the appropriate icon type.
In the example I used the icon from the VisualEditor, but it is likely to be slightly reduced.