https://sims4studio.com/thread/13120/cut-numbers-easy-explanation

Introduction

Cut numbers are what Sims 4 Studio uses to determine which mesh group in your .blend will be used to replace an EA mesh group in the original LOD. If the EA item you cloned has multiple mesh groups, you need to have a corresponding mesh group for each and your mesh group must have the cut number of the EA mesh group you want it to replace. If you are missing a cut number, Studio will not replace the EA mesh group that has that cut number, and it will stay in the package, show in the Studio model viewer, and be visible in the game.

This tutorial explains this a bit more and shows how to change the cut numbers of your mesh groups to match those of the original EA item. There are a few troubleshooting tips at the end, also.

This tutorial assumes you know how to clone an item and make your mesh. It will not show you how to do those steps.

What you will need

Sims 4 Studio (any current version) and Blender (2.70, 2.76, OR 2.78)

Instructions

Clone your item and export the LOD 0

Open the resulting .blend in Blender. You will find a list of all mesh groups in the original EA item on the right side. The dress I cloned has four mesh groups. Click on one of the mesh groups to select it. This will cause the cut number of that mesh group to show in the Cut field below. In the picture below, all the mesh groups are enclosed in the white box. The green arrows are pointing at the selected mesh group (s4studio_mesh_1) and its cut number (0003).

Please note, these numbers exist in the original item and they were selected by EA. For whatever reason, the cut numbers are not consistently assigned to the mesh groups. What this means is, in some items mesh group one may have a cut number of 0003 as in this case, but in others, mesh group one may have a different cut number. The only way to know the cut number for sure is to check.

Also of importance, different LODs in the same item can have different numbers of mesh groups, and the cut numbers of those mesh groups may vary. You need to export each LOD and examine it to know how many mesh groups it has and the cut numbers of its mesh groups.

Now that you have determined the cut number for mesh group one, you need to find out what part of the item mesh group one is. Selecting it by clicking on its name will highlight the mesh group in orange in the 3D view (Blender's model viewer). If that is not clear enough, you can also hide all the other mesh groups by clicking the little eye to the right of the mesh group name leaving only mesh group one showing. In the picture below, the white box encloses all the little eyes that have been clicked to hide mesh groups, and the green boxes enclose the only unhidden mesh group and the corresponding mesh in the 3D view. Now it is absolutely clear what part of the dress and Sim body correspond to s4studio_mesh_1. It consists of the torso, arms, waist, hips, thighs, and the uppermost part of the skirt.

This step can be repeated for each of the mesh groups in the original item. To un-hide a hidden mesh group, just click on the eye again. The picture below was taken after I hid mesh group one and un-hid mesh group two. This mesh group has a cut number of 0002 and consists of the part of the skirt directly below mesh group one.

Once you've gone through the original item and determined the cut number for each mesh group, you're ready to start adding your mesh groups to the .blend and removing the EA mesh groups. In the picture below, I've added a part of my new dress. On the right side, you can see the name of my new mesh group (Main_Dress_Mesh) enclosed in a white box. The green boxes enclose the EA mesh group I have selected and its corresponding Cut field. As shown above, this mesh group is the main dress mesh of the EA item (torso, arms, waist, hips, thighs, and uppermost skirt). The mesh I've named "Main_Dress_Mesh" also contains the majority of my new dress and goes from the torso to just below the knees.