All the capabilities that are populated to extend your clients’ capabilities are either created from WordPress, your theme, or your active plugins. You can use the extend client capabilities and remove menu items to limit your clients access to specific elements of their install.
From the WordPress codex here is what level 0 to 10 means. These are presets to give your client access to different capabilities with one click:
User Level 0 converts to Subscriber User Level 1 converts to Contributor User Level 2 converts to Author User Level 3 converts to Editor User Level 4 converts to Editor User Level 5 converts to Editor User Level 6 converts to Editor User Level 7 converts to Editor User Level 8 converts to Administrator User Level 9 converts to Administrator User Level 10 converts to Administrator
