How to model details of a system processing in bpmn?

Hi guys
I have a general question about some BPMN concepts (not the camunda tools) and i will be appreciate if you help me find the answer.
I want to model a process which contains a part that should be done by user in a system.
This part has a lots of steps. For example user should first sign up to system and he should fill a form for that. I want to model also the validation process of the form in my BPMN process (because i need to document the whole process in details) . for example check if the username is unique, the password has the required length and a few more rules. please consider that this parts should be in details i mean all the conditions and possibilities should be model in details.
My question is that how i should model this parts in my BPMN diagrams ? Do i should still use BPMN for that or i have to use some UML behavioral diagrams such as state machine or activity diagram for this purpose ? If No, how to connect my BPMN models with those UML diagrams and is this kind of modeling (mixing BPMN and UML together) valid and standard ?
And if Yes, how to model behavioral details of a system in details with BPMN ? for example checking that if a text field for phone number contains only numeric values should be a service Task ? and for containing Lane of this Task what should be the name of Lane ? “System” maybe ?
Thank you all very much

Hi @developer,

see my answer in the forum:

Hope this helps, Ingo

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To avoid confusion I’ll lock this post as it is being answered somewhere else.:slight_smile: