The panel

Open it from Proto Tools ▸ Proto Spline Volume (also under Window, Level Editor category).

Controls

Draw Spline / Confirm / Cancel with live status text.
Settings for the draw tool or the selected volume, plus a Spline Point section for the selected point's smoothing.
Volume list with sortable Item Label / Type / Points columns, search, inline rename (double-click), 👁 eye icons for per-volume visibility, and double-click to frame a volume in the viewport.
Filter funnel (left of the search box) shows or hides every volume of a type in the scene.
Multi-select volumes to edit their settings together.
? button (bottom right) lists every control and hint.

Selection is synced both ways with the level, and the list updates itself as volumes are added, deleted, duplicated, or undone.