I prefer to write
optional aligned with
Form Label. Does writing "not required"? sounds more clear to user
There are few reasons to prefer optional but they do not make its usage mandatory.
First and most simple because it is shorter then it requires less screen space. This has two advantages:
- It's easier to fit on (overpopulated) forms. Think that, despite all its drawbacks, required fields are marked with an asterisk mainly because of this.
- It uses less ink for 2nd class information.
Optional is a common well-known word understood by almost everyone (also not native speakers, some languages even borrowed this English word.)
- It is the antonym of required then it requires less cognitive load than a negation.
Plus two minor not widely accepted concerns:
- Positive wording is preferred because it produces a better feeling on users. Optional literally communicates that decision is left to the person choice, it makes the user in control while not required does not enforce the positive meaning you're trying to communicate marking optional fields instead of required ones (on contrary it mitigates this effect.)
- It might cause the field to be filled anyway (I remember I read something about this comparison but I'm not sure where) but this is pretty aleatory because location, native language and other factors will influence how user will perceive this. Sometimes optional might be perceived as nice to have (in Italian, for example) while not required has not this nuance.
Generally I see "Optional" more often than "Not Required".
I think the former is a better approach. The user has the option of entering the info- this gives them a stronger sense of choice/control over the data they're sending you. I think it's more preferred than making the user feel they're required to do something.
I find that this article has a good point on why it is better to mark the optional form fields: "Always Mark Optional Form Fields Not Required Ones".
In my opinion, the asterisk for the required inputs is the intuitive, simple, elegant solution for this information, I don't think it creates a lot of visual noise if it is not red and I prefer this option.
If you will want to mark the optional fields in the form, I think you should keep "optional" because it is clearer than a negative sentence / word.
Another consideration is the proportion of required vs optional fields. If there are few required fields and many optional, it is more sensible to make the required ones obvious. In all cases, negations ("not required") are less understandable.
I used to do programming in the Real Estate industry, and in some localities the forms had many fields but only a very few required, making it easy to "shoot first and ask questions later" (add a Listing and then fill in details when more is known). Other localities tried to get Agents to add as much detail as possible so that the data was more searchable, and more useful historically. There were also legal requirements about how long someone could "sit on" a Listing without adding it to the system. So there are issues of contract requirements and later use, not just the initial data entry.
To straight forward answer your question I would suggest for Label(Optional).
Also you can check this answer which relates to your query.