It should be possible to customize a priority for the Upstream DNS-Provider. I want to pick the custom Upstream DNS-Provider at first, before the preset ones will be used.
The preset Upstream DNS-Providers are on higher priority as the Custom Upstream DNS-Provider. If i insert a preset Upstream DNS IP-Address in a custom DNS-Provider field, after saving the custom field would be empty and the associated preset ones will be activated.
There is no priority for upstream DNS servers.
Pi-hole will make an effort to prefer the one that is providing replies the fastest, see also DNS resolver - Pi-hole documentation.
Thanks for your answer.
But it would be helpful for me to choose the priority.
Because, the main upstream DNS server should be my local DNS server (DC).
Only in case of no connectivity to the local DNS server, Pi-hole should provide another DNS server.
In case of local DNS servers, you'd usually want to involve them for local name resolution that is otherwise unknown to Pi-hole.
Pi-hole's Conditional Forwarding at the bottom of Settings|DNS is intended to address that.
It would serve to forward queries for local names (and only those) to another local machine with a DNS server.
Did you yet consider enabling CF?
Thank you very much for your answer.
I have not seen the Conditional Forwarding, since today.
My fault, sorry for that.
So i configrued CF and everything runs as intended. 
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