What it's supposed to do is create a new copy of the selected object, rotated to the same angle as the prim you click on, and positioned flush against that prim (i.e. no gap between them). In the past, this has been extremely valuable, allowing builders to quickly and accurately align prims against each other (such as the floors and walls of a house).

However, it does not work reliably when one or both of the prims involved have been rotated away from the default <0,0,0> rotations.

• If one prim has been rotated, the copy will have the correct new rotation, but its position will be incorrect. This bug has existed for as long as I can remember, as far back as Sept 2006.
• If both prims have been rotated to different orientations, neither the new rotation nor position of the copy are correct. This misbehavior is new, within the past several months.

(Incidently, it does work correctly if both prims have been rotated to the same orientation. This fact made the longstanding bug minor, as making a copy of the copy resulted in the correct behavior.)

REPRO #1: (To see what it's supposed to do all the time)
1. Rez two cubes, A and B. Do not rotate either cube.
2. Right click on B, and Edit it.
3. Access the Create tab of the Build floater (Ctrl-4), and enable the checkboxes "Copy Selected", "Center Copy", and "Rotate Copy"
4. Click on a face of A.
= Observed and expected: a new copy of B appears, with the same rotation as A, positioned flush against A.

[[ NOTE: I accidently switched the labels in the pictures for repros #2 and #3, so I'm also switching the A and B designations below ]]

REPRO #2: (Rotation is correct, but position is not)
1. Rez two cubes, A and B.
2. Rotate A to a rotation different from B. (The more different the angles, the more dramatic the bug.)
3. Right click on A, and Edit it.
4. Access the Create tab of the Build floater (Ctrl-4), and enable the checkboxes "Copy Selected", "Center Copy", and "Rotate Copy"
5. Click on a face of B.
= Observed: a new copy of A appears, with the same rotation as B, not positioned flush against B.
Expected: a new copy of A should appear, with the same rotation as B, positioned flush against B.

REPRO #3: (Neither rotation nor position is correct)
1. Rez two cubes, A and B.
2. Rotate both A and B, but not to the same rotation. (The more different the angles, the more dramatic the bug.)
3. Right click on A, and Edit it.
4. Access the Create tab of the Build floater (Ctrl-4), and enable the checkboxes "Copy Selected", "Center Copy", and "Rotate Copy"
5. Click on a face of B.
= Observed: a new copy of A appears, with a different rotation than B, not positioned flush against B.
Expected: a new copy of A should appear, with the same rotation as B, positioned flush against B.