Once you have your Woodland compartments project created, please see this guide, you can duplicate this project. To do this click on the 3 vertical dots next to your Woodland compartment project and select Duplicate.
Enter your new project name and select Create Project.
Remember to switch off the visibility of your original project so you can now only see your new one.
You can now use the spilt tool to create sub compartments. To do this right click and select the split tool.
Start your cut outside of the area you wish to split and finish outside with a double click. The feature will now be split. Continue this process until you have all of this splits you need.
If you now want to add felling or thinning details to your sub compartments just select the feature and the Feature Details menu will appear on the right. You will also be able to change the colour styling to distinguish between the sub compartments should you wish.