"En planta" normally refers to a plan view or floor plan, so I imagine this means what you see when looking at the pallet from above; in other words, the length by the width. Here's a ref. about how pallets are measured:

Pallet dimensions- Measurement points to determine a pallet's dimensions:
Length=distance between pallet ends (inclusive of overhang) and parallel to the stringer(s).

Width=distance between pallet sides at right angles to the pallet length inclusive of any overhang

Height=distance from the outer edges of the bottom deckboards to the outer edges of the top deckboard.

http://www.mhia.org/keywords/keyworddescr_template.cfm?keywo...