Separate necessary from desirable.
A requirement describes something that must be true for an approach to be usable. A preference describes something valuable that may be negotiable. Keeping them separate makes a discussion more precise.
In an illustrative visitor check-in situation, confirming that an arrival was recorded might be necessary. A particular screen arrangement may be a preference. Ask why each condition matters and how it could be recognized in practice, rather than treating every suggestion as equally essential.
Record the limits around the work.
Access to people, equipment, time, or information can affect what an investigation can establish. A limited setting may be appropriate, but its boundary should be agreed before the result is interpreted.
For example, observing a quiet reception period does not establish what happens during a busy arrival window. Record which conditions were represented and which were not. Do not disguise a practical restriction as evidence that an unexamined situation will behave in the same way.
Keep unresolved points explicit.
Some conditions will remain open. Give each important open question a clear description and identify what information would help resolve it. This is more useful than silently choosing an assumption and allowing it to become a requirement.
Revisit the conditions when the situation changes. A new location, another user group, or a different operating arrangement may alter what matters. Requirements are a working account of the need, not a reason to ignore new information.