The R12 file system changed significantly from 11i. It now features a dual file system (fs1 and fs2) to support (introduced in R12.2). $APPL_TOP: Contains the product directories and files.
Stores instance-specific configuration and log files. Oracle Apps R12 Technical Tutorial Pdf
If you are looking for deep-dive tutorials, focus on these official Oracle documentation categories: The R12 file system changed significantly from 11i
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Creating validation rules for report parameters.
Being phased out but still present in many modules.