Decal Gizmo

When an active decal object is selected, a decal gizmo is displayed around it. It consists of the following elements:

Move

Move: the dashed rectangle around the decal. When you drag it, the decal will automatically snap (and get parented) to the object under the mouse; the new parent and the currently selected objects will also be set as the decal's projection targets (the original projection targets will also be included if the new parent is among them). The decal's Z-axis always aligns to the surface normal, but its rotation around that axis is determined by one of the following modes:

Current: uses the shortest rotation from the current decal orientation that aligns Z-axis with the normal
Initial: uses the shortest rotation from the initial (at the start of the dragging) decal orientation that aligns Z-axis with the normal. This mode is indicated by a dashed line between the decal's initial and the current position
Upright: decomposes the decal rotation into pitch, yaw and roll ("YXZ Euler" in Blender), and changes pitch and yaw to align Z-axis with the normal, while keeping roll intact. This mode is indicated by a dashed line extending upwards from the decal's current position
Edge: aligns Z to normal, and X or Y axis (depending on which is closer) to the direction of the nearest edge. This mode is indicated by a dashed line oriented along the nearest edge
Pressing Tab will switch the surface alignment mode (it will cycle between the available modes)
Holding A or Middle mouse button will temporarily toggle on/off the alignment to the nearest edge

Rotate

Rotate: the curved icon at the corners of the gizmo. Rotates the decal around its Z axis

Shift (hold): rotate by 45° increments
Ctrl (hold): rotate by a larger increment (Rotation step in the preferences)
Alt (hold): rotate by a smaller increment (Rotation substep in the preferences)
If Snap rotation by default is enabled in the addon preferences, holding no modifier keys will rotate by a smaller increment, and holding Alt will rotate without snapping instead

Scale

Scale: the angular icon at the corners of the gizmo. Scales the decal uniformly. If Alt was held at the start of the operation, either the "XY range" / relative size (for material-based decals) or the scale adjustment parameter (for object-based decals) will be affected instead of the object scale.

Shift (hold): scale by 0.01 increments.
Ctrl (hold): scale by 0.1 increments.

Flip

Flip on axis

Flip X: the triangle on the right side of the gizmo. Flips the decal along the X axis
Flip Y: the triangle on the bottom side of the gizmo. Flips the decal along the Y axis

Curvature

Curvature: the icon on the left side of the gizmo, shaped like the Greek letter Ω. Dragging the mouse will change the decal's X and Y curvatures

Shift (hold): change by 0.1 increments

Project

Project: the grid/hash icon on the top side of the gizmo:

Click toggles on/off the decal's projection mode (if projection is enabled, a couple of additional icons are displayed nearby)
Ctrl+Click toggles on the projection mode, adds the selected objects to the list of projection targets, and recalculates the projection volume depth from the distance to projection targets' nearest surfaces
Shift+Click toggles the decal's embedding mode (if embedding is toggled on, projection mode gets enabled too). If the decal is in embedding mode, the grid icon is displayed at a 45° angle

Depth

Depth above / Depth bellow