A text field may offer a small list as you type and still accept a value absent from that list. Suggestions and enforced choices are different interface behaviours.
MDN’s datalist documentation explains how suggested values can be associated with an input. It is not simply an interchangeable version of a select menu.
Test the rule with an unlisted answer
For a fictional practice form, suggest three room names while leaving a text input available. Enter a fourth name. Then check the form’s validation and the server’s rules separately: suggestions alone do not define the complete acceptance policy.
If only three rooms can actually be booked, communicate that restriction clearly and use an appropriate control. If visitors may name another location, explain how those entries will be handled.
Browser presentation and accessibility support can vary, so test the intended experience with keyboard navigation and relevant assistive technology. Do not assume a list that appears on your computer behaves identically for everyone.
As a visitor, distinguish “choose one of these” from “start typing, or enter another value.” When the instruction is unclear, an unlisted answer may require clarification rather than repeated attempts to make it appear in the suggestions.
Editorial illustration from this site’s image library; not documentary evidence of the example or object discussed.