It may be helpful if the search tips would be extended by more lines, telling details about how the search terms are processed. This isn't clearly communicated.

Proposal:
* Enter keywords into the desired fields, then press the "Search" button.
* The Search looks for posts, hence a thread may be represented multiple times.
* The search fields are AND-combined, which means the results must match at least one word from each field.
* For an OR-search over more than one search field do a separate search for each needed field.
* Alternatively use the General field to search all available text fields (i.e. the [Post-]No. field is not included).
* The keywords inside the individual search fields are OR-combined, returning every post matching at least one word.
* You can choose to match whole words or let the search work like there are wildcards at the end of the words.
* Quotes around search terms have no effect.
* Excluding by preceding a keyword with a dash ("-") is not possible.
* You can choose to look only for opening posts.
* You can select the boards from which posts are returned.

This is what I found by trying. Of course I have no idea if it is complete and/or correct. Does a more detailed guide exist? Provided with an even technical manual I'm willing to write a "readable" version.