Summary

In CLO 3D, we see patterns and avatars going into each other like in the picture below. The undesired penetration between 3D objects is called Collision. There are two ways to fix it: re-arrangement tools and mesh manipulation.

Re-arrangement tools
Right click on the 3d pattern and in the property editor you will find the re-set 2d arrangement selected and the re-set 3d arrangement selected.

Reset 3D arrangement will bring the arrangement of selected Patterns back to the pre-simulation state in the 3D window.

Re-set 2d arrangement will re-arrange selected Patterns in the 3D window identical to the 2D window state.

You could also find arrangement tools in your 3d toolbar. There are Re-set 2d Arrangement All, Re-set 3d Arrangement All and Re-drape 3d Arrangement. They will do the same job to all patterns instead of the selected ones.

Mesh Manipulation
You can select parts of the mesh on a whole pattern to edit the selected mesh separately. There are Select Mesh and Pin. Brush, box and lasso are different methods to select meshes. Selected areas will be displayed in green.

Select Mesh (Brush)
Left click and move your cursor on your 3d garment, the meshes you went by will be selected and highlighted in green. Right-click on the green area to subdivide, create a pin, freeze and solidify the selected mesh.

Select Mesh (Box)
Click and drag on the Pattern to create a marquee box to select the desired area. You can click and hold your clicking on that area to move it manually or single click to get this gizmo to move it in 3D.

Select Mesh (Lasso)
Click and drag on the Pattern to freehand Lasso selection as desired to select mesh. You can click and hold your clicking on that area to move it manually or single click to get this gizmo to move it in 3D.

Pin (Box) & (Lasso)
Pin will fix selected parts of the 3D garment during simulation. You can click and drag pins on the 3D garment to desired positions. Pins are displayed in red.