Mechanism: a tapered plunger closing gradually into a matched conical seat
"A needle valve has a relatively small orifice with a long, tapered seat, and a needle-shaped plunger on the end of a screw, which exactly fits the seat." Turning the screw retracts or advances that plunger, and "as the screw is turned and the plunger retracted, flow between the seat and the plunger is possible; however, until the plunger is completely retracted, the fluid flow is significantly impeded" — the seal only fully releases at the very end of the stroke. The fine-threaded screw is what gives the valve its resolution: "since it takes many turns of the fine-threaded screw to retract the plunger, precise regulation of the flow rate is easily possible" — a geometry a quarter-turn ball valve or a shorter-travel globe valve cannot reproduce at the same bore.
Industries served: protecting instruments from surge, and metering flow too small for a general-purpose valve
"All field analog instruments are fitted with a needle valve to control flow entry, where sudden surges of fluid under pressure can damage the instruments" — the valve's job on an instrument line is as much protective as it is about flow-setting. The same fine-throttling geometry is specified "in all industries for controlling and metering fluid applications such as steam, air, gas, oil, water, or other non-viscous liquids," and at sample points in piping "where the flow must stop gradually and... precise adjustments of flow are required." Outside process plant, the same geometry shows up wherever a small, steady flow has to be dialled in and held — Wikipedia cites "vacuum systems, when a precise control of gas flow is required, at low pressure, such as when filling gas-filled vacuum tubes, gas lasers," and pressure-gauge shutoff duty. That range, instrument protection at one end and precision metering at the other, is consistent with Himalay's own catalogue sitting almost entirely in threaded, bar-stock, high-pressure configurations rather than larger flanged process sizes.
Standards landscape: an instrument-valve design standard, plus the thread standard that governs how it connects
MSS SP-99 governs instrument valves — needle, packless, ball, plug, check and manifold types — for steel and alloy construction at NPS 2 and below, rated to 15,000 psi at 100°F. ASME B1.20.1 sits alongside it for the threaded-end connections: it is "the foundational Standard for NPT, NPSC, NPSM, and NPSL pipe threads," covering "dimensions and gaging of the world's most common pipe threads," used, in ASME's own framing, in everything "from critical applications demanding robustness and precision to common plumbing and hardware." On a needle valve, thread dimension and gaging matter directly: a threaded NPT connection is a tapered, sealant-dependent joint rather than a parallel thread, so gaging tolerance affects whether the joint holds at the pressures this family is rated to. Sources: [Wikipedia — Needle valve](https://en.wikipedia.org/wiki/Needle_valve); [Hardhat Engineer — Needle Valve: A Complete Guide for Engineer](https://hardhatengineer.com/needle-valve/); [ASME — B1.20.1 Pipe Threads, General Purpose, Inch](https://www.asme.org/codes-standards/find-codes-standards/b1201-pipe-threads-general-purpose-inch)