I have since resolved this  by following the guidelines here:

https://www.zoho.com/creator/help/fields/configuring-conditional-drop-downs.html

I have created an order entry form based on the New Order Form in the Order Management application.

In time we could have many hundreds of products and the prospect of scrolling through these in the Product Name field looking for a single product will be somewhat daunting. Image Add Product

At the time of loading a new product we also assign it to a Product Group and a Product Brand. Image New Product

I would like to include the Brand dropdown in the Add Product subform and when a specific brand is chosen, only the products associated with that brand will be listed in the Product Name dropdown list.

I may have read about this somewhere before (contents of  a picklist determined by selection in another picklist??).

Would appreciate advice on how to do this