When designing forms it is very important to ensure that the tab order is logical / correct. From a user's perspective it is very frustrating when this does not occur.
When the user enters data into a field on a form then presses the TAB key, the focus is moved to the next designated field. Usually the tab order is the order in which the fields are displayed on the form. However, this can be changed if required, e.g. data being entered may be taken from a hard copy form - when reading the data from the form, it is useful for the user to enter the data based on the order of data on the hard copy form.
Also, adding a new field to the form may render the form's tab order incorrect; the tab order should then be corrected.
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or press CTRL + ENTER. 
Suggestion: Ensure that each control is named appropriate to the associated field data, i.e. avoid default names such as Text55, Combo22. For forms containing a large number of controls, appropriate naming will make changing the Tab Order an easier process.
Related Tip:
Access Tip # 3: Assigning A Keyboard Shortcut To Form Fields/Controls