Z?2f0i1e2l0d914

z?2f0i1e2l0d914 (called on the JASRAC database) is Disc 2, Track 8 on the XenobladeX Original Soundtrack. It plays as the area theme in (most) caves: It is one of the few area themes that takes priority over Don't worry.
 * The Beast's Lair in Primordia.
 * The Castaway Cavern in Primordia.
 * The Headwater Cavern in Primordia.
 * The Molten Hollow in Primordia.
 * The Rock Cavern in Primordia.
 * The Stonelattice Cavern in Primordia.
 * The Turtle Nest in Primordia.
 * The Ensanguined Font in Noctilum.
 * The Rockmole's Burrow in Noctilum.
 * The Sato Headwater in Noctilum.
 * The Shark's Jaws in Noctilum.
 * The Vitriol Cesspool in Noctilum.
 * The Whale's Gullet and Whale's Nostril in Noctilum.
 * The North Ant's Nest, Ant's Nest, and South Ant's Nest in Oblivia.
 * The Beachside Trove in Oblivia.
 * The Demon's Pocket in Oblivia.
 * The Devil's Colony in Oblivia.
 * The Rooney Cavern in Oblivia.
 * The Banshee Cave in Sylvalum.
 * The Behemoth's Shadows in Sylvalum.
 * The Quay Hollows in Sylvalum.
 * The Secluded Lava Lake in Sylvalum.
 * The Bestial Utopia in Cauldros.
 * The Infernal Ledges in Cauldros.
 * The Old Ceremonial Hollow in Cauldros.

Etymology
The track title can be split into 3 parts: the prefix 'z?', the digits '20120914', and the letters 'field'. The letters likely indicate that this is music heard in the 'field', or area music. The digits can be interpreted as the date 2012/09/14, or the 14th of September 2012, the date in which the track was recorded or mixed.

The prefix 'z?' is similar to other tracks that start with 'z' and a number (possibly followed by 'b'). The significance of this is that the composer Hiroyuki Sawano gives codenames to tracks before he properly titles them: a letter/abbreviation followed by a number, where the letter/abbreviation corresponds to the soundtrack in question. In the case of Xenoblade Chronicles X, this letter was 'z' (Xenoblade X's Japanese name, ゼノブレイドクロス, can be romanised as Zenobureido Kurosu). The number corresponds to the ordering in which tracks were composed. 'z?' is likely a holdover from this codename (perhaps the exact ordering in which this track was composed was forgotten).

Therefore, 'z?2f0i1e2l0d914' decodes into 'The ?th track composed for Xenoblade X, a field theme, recorded/mixed on the 14th of September 2012'.