Input a fixed number for use by conditional statements, a constant?

Are you working on this form https://www.jotformpro.com/form/82236026051951?

If I understood the concept of your form and your question correctly, you want to charge your users $15 + $15 for each additional registrant that they add.

Here is a demo form that I created - https://form.jotform.com/82244083145957

Feel free to test it and clone it.

In my form, I set the default value for the field "TempFee". I also set this field to be read-only. 

I added "How many - Additional registrant(s)" field and I assigned calculation values to this field.

How to Assign Calculation Value

Then I set a calculation - "Fee $" field.

How to Perform Form Calculation Using a Widget

And I passed the calculation to the payment field.

How to Pass a Calculation to a Payment Field 

I also set several conditions to the Additional registrant(s) - name fields.

How to Show or Hide Fields Base on User's Answer

Hide or Show Multiple Fields at Once

Hope this meet what you are looking for. 

Thanks much for your prompt response – judging from the link names, it seems that you’ve captured my intent.
I’ll go through the links and contact you again if I have problems.
Thanks again,
Charles
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Can you tell me if it’s possible to write repeatedly to a multiple type field, adding to the field and forming a list? For example if a user input field is filled, write its value to a cumulative field where it would be added to a list, or an external file.
Thanks
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I am not sure if I understand you correctly.

If you want to add multiple participants in a list, you can try using the Infinite List Widget.

DEMO: https://form.jotform.com/82256684050962

The form built by my colleague seems to fulfill your request as well.
Form URL: https://form.jotform.com/82244083145957

Feel free to clone any of the forms and adjust them to your needs.

How-to-Clone-an-Existing-Form-from-a-URL

If you meant something else in your last reply, please give us more details on what are you trying to achieve.

Thanks, the infinite list widget looks promising. I actually just wanted to accumulate all the names on a single list, printable, or to an external file.
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I actually just wanted to accumulate all the names on a single list, printable, or to an external file.

For an external file, you can any of these formats: XLS, CSV or PDF.

Complete guide: How-to-Download-Form-Submissions-as-Excel-CSV-PDF

To print your submission data, you can either utilize the PDF SUBMISSION REPORT, or just click the PRINT button right on the SUBMISSIONS PAGE.

My recommendation is to just export your data to XLS or CSV format so you can achieve all three - You will get the name list on the sheet, you can print it through Excel and save it as an external file.