For example, the user starts to fill out a form and then wants to save and finish filling it out at a later time.
Users will have to use either Acrobat or Reader to view XFA forms you've created with Designer.
Third party PDF viewers generally won't support those files.
if your users using Acrobat they can save changes to the forms as often they want.
But if they are using Reader, you have to add additional rights (Reader Extenstions) to your form first.
Please use the search function in the forums, there are countless threads about this topic.
PS: There is also a auto-save feature you can enable with LiveCycle Designer ES3 or higher in your forms:
Adobe LiveCycle ES3 * Run-time (Form Properties dialog box)
Thanks for the reply. Just to be sure, when forms are created with Designer, the user will be viewing them in either Acrobat or Reader.