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They are rig dimensions primarily used by sailmakers to know how to size a sail. Loose definitions of them are:
I= distance from deck to where the forestay attaches to the mast
J= Distance on deck from forestay to the mast
P=Distance from top of mast to boom.
E= Length of boom.
I say "loose" because the actual dimension may be (for example for P) to a black line at the top of your mast, which is the highest point the main will be raised to.