What are the rules on form field names? Eg. Can they have spaces?

At Structure of a form definition in the DataForms topic, there is a note listed under "Field Name Name Notes":

/System/DataForms"> Field Name Notes:

• Field names have to be unique.
• A very few field names are reserved. If you try to use one of these names, Foswiki will automatically append an underscore to the name when the form is used.
• You can space out the title of the field, and it will still find the topic e.g. Aeroplane Manufacturers is equivalent to AeroplaneManufacturers.
• If a label field has no name, it will not be shown when the form is viewed, only when it is edited.
• Field names can in theory include any text, but you should stick to alphanumeric characters. If you want to use a non-wikiname for a select, checkbox or radio field, and want to get the values from another topic, you can use [[...]] links. This notation can also be used when referencing another topic to obtain field values, but a name other than the topic name is required as the name of the field.
• Leading and trailing spaces are not significant.

If you would like to customise the field name only visually (while keeping the FieldName constant), this is not possible with a default Foswiki. The recommended alternative is to make use of FlexFormPlugin and an auto view-template.