If I import a STEP file, can I ever get the feature tree back?

No — not unless someone also sends you the native file. Once a model has been exported to STEP or IGES, the original sketches, extrude depths, and fillet radii are gone from that file; some CAD systems can auto-recognize features (fillets, holes, bosses) on an imported dumb solid to make it easier to edit, but that is a best-effort reconstruction, not the original parametric history.

Is Parasolid (.x_t/.x_b) a native or neutral format?

It is a middle case. Parasolid is a geometric modeling kernel used natively inside several CAD systems (including SolidWorks and NX), so a .x_t file carries a clean B-rep solid similar to STEP — but because several major CAD products share that exact kernel, a Parasolid export can sometimes retain more fidelity between those specific programs than a STEP export would. It still does not carry a generic feature tree the way a native SLDPRT does.

Why does my IGES import show tiny gaps or fail to "sew" into a solid?

IGES represents a part as independent trimmed surfaces rather than a topologically closed solid, so numerical tolerance mismatches between adjoining surface edges are common — the receiving CAD system has to stitch ("sew" or "heal") those surfaces back together within a tolerance, and sometimes cannot close every seam automatically. STEP's explicit solid topology makes this failure mode far less common, which is the main practical reason STEP has largely superseded IGES.

Should I export STEP AP203 or AP214/AP242?

For basic geometry exchange, AP203 is the oldest and most universally supported. AP214 added color, layer, and some PMI support aimed at automotive workflows. AP242, the current combined standard, is the best choice when the receiving system needs semantic PMI/GD&T, assembly structure, or long-term archival — most modern CAD systems default to it or offer it as an explicit export option.

Does exporting to STEP always lose accuracy?

Not meaningfully for geometry — a well-formed STEP export preserves exact analytic surface definitions (planes, cylinders, NURBS) to the tolerance the exporting system used, so the resulting solid is not an approximation the way a mesh (STL) is. What is lost is not geometric accuracy but parametric structure: the "why" behind the shape, not the shape's dimensions.

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Native vs. Neutral CAD Formats (STEP/IGES)

Why exporting to STEP or IGES doesn't hand the next engineer an editable copy of your model — it hands them the shape, not the recipe that built it.

A mechanical engineer emails a SolidWorks part to a supplier who runs CATIA, and is surprised when the imported file shows up as a single gray blob they can't edit — no sketches, no extrude, no fillet parameter to bump from 3 mm to 5 mm. Nothing went wrong. That is exactly what a neutral CAD format is supposed to do: carry the finished shapeacross a vendor boundary that has no shared understanding of how either company's CAD kernel builds features. The feature tree — the sequence of sketches, extrudes, fillets, and patterns that make a part editable and parametric — is native-format-only. STEP and IGES were never designed to carry it.