obligatory

attachable, adjustable, detachable, inbuilt, mandatory, automatic, selectable, obligatory, removable, keyless, stowable, compatible, customizable, lockable, sunblind, retractable, compulsory, waterproof, enhanced, mountable, antilock, interchangeable, standard, equipped, expandable, threadless, gusseted, extra, cocurricular, fillable, superfluous, complimentary, available, immersible, integrated, omnidirectional antenna, additional, zoot, de rigueur, sealable, variable, adaptive, subminiature, customised, insertable, octal, assignable, unlimited, ductless, continental breakfast, data encryption, antistatic, alphanumeric, scart, outboard, porterage, handy, nonmandatory, closable, push button, foglamp, programmable, cupholder, daisy chaining, rainproof, plus, underslung, inline, unmounted, foldable, elasticized, error correction code, preset, manual, ripstop, weatherproof, astroscope, phosphor bronze, countersink, reciprocating saw, lite, chamfered, backward compatible, transmissive, spline, flanged, extendable, actuated, lavalier, automatic transmission, unthreaded, microphonic, nonslip, detent, rearmost, essential, shiftable, baud rate, pricey, sueded, preassembled, cleanable, designator, filename, barrett, centrifugal, encryption, ff, initiators, keyboard, operand, operands, specifier, token, \u00bb, animator, arbitration, beforehand, boolean, cas, catalyst, clark, instantiated, checking, nonessential, reusable, supplied, declarative, indispensible, required, standardized, generic, specialized, standalone, configurable, delete, indicative, predesigned, semiempirical

extension, filename extension, file name extension, optionally, punks

elective, ex gratia, facultative, nonmandatory, nonobligatory