..SourcePage is used to find out the source of an internal page (which means the internal page displayed by the Internal Page control).
Trace(IP_InternalPageControl1..SourcePage) // Displays IP_InternalPage2
<Name of Internal Page Used> = <Internal Page Control>..SourcePage
<Name of Internal Page Used>: Character string
Name of the internal page used by the "Internal page" control.
<Internal Page Control>: Character string (with or without quotes)
Name of the Internal Page control.