Hi,
The DT individual form is a HMRC form. Some versions are tailored to certain countries, such as Germany.
These forms for Germany can be found at:
Double Taxation: treaty relief (Form DT-Individual (Germany)
While it does not matter to HMRC whether the generic form or the country specific form is used, the other country involved may choose to reject the generic form in favour of the tailored form. We would suggest the tailored form.
Thank you.