From what I can tell, Splunk can use Booleans for internal logic (such as with "if" and "case" functions within eval , or as something to mark and count as with chart ), but it cannot return them, in the sense that you can't use the eval statement to generate a field that holds a Boolean value.
The documentation is not inconsistent, but confusing. They are overloading the term eval to be used in two different places, one as a standalone command, and one as a function that can be used as part of count within the chart command. In the first case, as it states, it cannot be used to create a field that holds a Boolean value. In the second case, it creates an internal Boolean state that can be counted into a field value. The documentation you are looking for isn't there because the documentation on the eval command is unrelated to the documentation on the chart command.
The answer to your question " count works with fields so it seems it returns field, but which one?" is that it may seem to return a field, but it doesn't - it is only capable of creating an internal Boolean state that Splunk can count.
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