What is the "value" field in Field Settings & Mapping?

[gjz]

I am trying to set up an Indexed Source and am having trouble creating the Field Settings & Mappings.  I don't understand what should be entered in the value field and I can't find any documentation or community posts that explain it.

For example, the Indexed Source for Requested Item has three fields settings, including one for the field "description".  We rarely have data in "description", I want to add "short_description".  I set the "Value" field to text, the same as the description field's value, but it gave me an error.

Where can I find what the valid values in the "Value" field and how it's used?

[Eric Pleines]

I believe this article will provide some insights:

Field settings (servicenow.com)

Here a quick example on how we are using it:
Besides our KBAs in ServiceNow we also wanted to include some external articles in our search.
After we ensured that the external articles are available in ServiceNow, we had to map the attributes of these external articles to the values "text" and "title" to ensure they can be found and will be displayed the same way as internal articles. So we set: Field = "ExternalArticleTitleAttribue"; Attribue = "MapTo"; Value = "title". This way the attribute of these special items that contains the title will be handled the same way as the short description of a normal KBA.
There are more operations you can do than just "MapTo", like make fields "not_searchable". The article describes the options in more detail.