AvanturinA good description talks not about technology but about what should change. Write who will use it, what they do now and what should become easier.
Add an example: one real client, one document or one working day. An example explains a task better than a page of requirements.
If there are constraints, state them right away: deadline, budget, services you cannot give up, data storage requirements.
It is fine if there is something you do not know. That is what the first conversation is for.