Click to expand/collapse this hidden text{=fieldname}
generates a null value if named field is not found on the current page.

Using {fieldname} just returns the most recently defined value for the named field. Using the = syntax enables a composite field to be defined which automatically resets its value to an empty string each time a page is started.

For example, if only one of FIELD1 or FIELD2 occurs on the current page then

{=FIELD1}{=FIELD2}

will contain the value of whichever field occurs.
Click to expand/collapse this hidden text{+=fieldname=['same_value'][,'different_value']}
This redefines the value of the named field, dependent on whether the field-value is the same as the previous page, or different.

+=fieldname=

Must start with += followed by a field name and an equals sign.

same_value

(optional)

The value assigned if the named field's value is the same as on the previous page – unchanged.
If omitted, the value is not redefined.

different_value

(optional)

The value assigned if the named field's value is the different from the previous page – changed.
If omitted, the value is not redefined.

When the same_value/different_value parameters are single 'words' they need not be in quotes, but note that # characters that are not in quotes indicate a 'Counter': see Numerical conditions.
•{?=fieldname=['same_value'][,'different_value']}
This is equivalent to the +=fieldname syntax above.

See also More on defining composite fields.