Every form must have a key field as determined by the key="" attribute within the form tag.
The key field records input from a user which must be unique in every entry (ie instance of a form). For example, the following shows a one question form with the key field indicated.
If a user enters a value which has already been recorded they will receive a warning
You can specify that a key field is autogenerated using the genkey="" attribute. and link this to an input, as in the following example:
In this case, a value will be automatically generated for each entry, consisting of the handsets IMEI number, concatenated to the current time in milliseconds, ensuring uniqueness.
For autogenerated keys, it is advisable to also use the hidden="true" attribute otherwise the user will see this string.
Keys becoe increasingly important when linking forms together as the key field from one form is included in the linked form.