I don't get the behaviour of custom field search

Dear Team,

I don’t get the point with custom field search.
It seams to me, no matter what I do, the search shows all custom posts.

What I want:
I have a custom post type with some fields. I am interested in:
Prozess
Dokumententype
and also I have tags.

I expected, that if I check a Prozess and / or a Dokumententype Ajax search will only performed in the custom post with these where the prozess and Dokumententype is the same.

Let’s say we have:

1 CP:
Title: “First Item”
Dokumententype: “Prozessbeschreibung (PB)
Prozess: “Marketing und Aquisition”
Tags: gesystag, gogo

2. CP:
Title: “SecondItem”
Dokumententype: “Prozessbeschreibung (PB)
Prozess: “Marketing und Aquisition”
Tags: gogo

If I uncheck “All” for both, ASP opens the search result and shows me
document-download (<— no idea, what this could be)
gesystag
gogo
First Item

If I enter “gogo” and uncheck all, I get “gogo” as a result.
Why is that? I have unchecked all Dokumententype and prozess

The same if I select different Dokumententype and prozess ….

I really hope, you get my point. It is a little bit difficult for me to explain ….

Thank you,
Michael