Define which triangles will be affected by a semi-automatic repair action
There are several commands available in the repair module to select triangles.
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Select Triangles: Selects individual triangles.
Select Surfaces: Selects entire surfaces.
Brush Selection: Selects individual triangles by painting over them with a brush.
Select Shells: Selects entire shells.

Left: Selected triangles. Center: A selected surface. Right: A selected shell.
In addition to these basic selection commands, you can quickly select all triangles, clear the selection, or toggle the selection.
Select all Triangles to select every triangle in the mesh.
Clear Selection to deselect all triangles.
Toggle Selection to flip the selected and unselected triangles.
Selection Primitive. Available options include:

Left: Drag a primitive shape. Right: The resulting selection.
If you have already selected triangles or surfaces, you can quickly expand or shrink the selection.
Expand Selection to include the adjacent triangles or surfaces in the selection.
Shrink Selection to remove the adjacent triangles or surfaces in the selection.
If you have a part that contains a hole, some actions benefit from, or even require, selecting one or more of the triangles directly touching the hole.
Select all Surrounding Triangles to select every triangle touching the hole.
Select Attached Triangle only to select the triangle adjacent to the edge.

In preparation for repair, the triangles surrounding the hole have been selected.