I'm not sure, but I think the job title might change from place to place. Like, at the Limited, I think they have a position called "Trend Forecaster" where they travel and try to determine upcoming trends in textiles, prints, and silhouettes. But, at other companies, sometimes they call that Planning, Merchandising, or some other title. I also think it depends on the size of the company.
A technical designer determines the correct "fit" of the garment. I think it's easiest to explain by going through the "life cycle" of a garment: first the Designer hands us a sketch of the design and tells us what fabric/yarn they want it made in and then the tech prepares a "Spec Sheet". the "Spec Sheet" contains the finished measurements (for a sample, we use size "M" or "8" in the missy market), the construction details (i.e. if we want single-needle topstitching around the neckline or double-needle topstitching, et.al), a "how to measure" diagram when necessary (but, if it's a basic, common style those usually aren't necessary). the tech sends out the "Spec Sheet" to the vendor/factory for them to make a "1st Fit/Proto" sample (sometimes the company also asks for a "showroom sample" for the sales staff or a "photo sample" if it's to be photographed for advertisements). When the 1st fit sample arrives from the vendor/factory, we measure the garment and then fit the sample in a fitting on a live model (the design team, production team also attend, other people in that product category can attend too), and discuss what needs to change (do we like the neckline? are there "drag lines" on the garment? do we like the sleeve length? do we want a tighter/looser fit?, et. al). Then, the tech sends out fit comments to the vendor to make corrections on the garment, and request a 2nd fit sample. We keep fitting until the garment fits nicely (sometime the 1st fit is perfect and we can go into production on that fit, but sometimes it can go up to a lot more fittings), then we go to a "Pre Pro" sample, where the factory is ready to start knitting/cutting, but we want to see it to make sure it's perfect. We issue the "graded specs" (we take the size "M" or size "8" specs, and then use math formulas to make the finished measurements for the balance of the sizes that we're producing), usually after the fit sample is approved, and request the vendor/factory to send us a "size run" (a sample in each size), which we measure only (to see if they met our specs) but rarely fit. That's where the life cycle of the garment ends in the technical design area. After that, it's in production and quality assurances area. Sometimes, though if there are big "quality assurance" issues on garment, we're called in to help see if we can help (but if the vendor made the garment 5" too short, sometimes they have to send it back to the vendor and we've missed potential sales).