Initiates after the form design and data are merged, the form exists, and the form’s layout is applied. At this time, the finished form has not been rendered; therefore, a calculation or script set to run on this event could modify the layout before the forms is rendered. This event also occurs after the form is rendered if a calculation or script changes the data or causes a change to the form in Acrobat or Adobe Reader.
You can use this event to perform tasks immediately after the form layout is established. For example, you can use this event to determine the number of pages the form contains.
See also 

layout:ready event