German engineering practice separates two documents, both defined in DIN 69901-5 and VDI 2519. The Lastenheft is written by the customer and states what is required and why — deliberately solution-neutral. The Pflichtenheft is written by the supplier in response and states how it will be built.
The what and the how are different documents, by different authors, in a fixed order, and together they form the contractual basis of the project.