Here comes the legal stuff.
Typically, a license notice should appear near the top of every source code file (just below the documentation. Most of the original source code from ClearFoundation is licensed under GPL or LGPL. If you are modifying and publishing the ClearOS source code, you must comply to the provisions of GPL/LGPL.
The underlying API/libraries in ClearOS are licensed under LGPL, while the core framework is licensed under the open source CodeIgniter license. This licensing makes it possible for developers to create proprietary apps that still use the core libraries from ClearFoundation. In other words, you do not have to use a GPL license if you call ClearFoundation libraries.
You can choose your own licensing model of course, so the following is just showing an example license notice.
/////////////////////////////////////////////////////////////////////////////// // // This program is free software: you can redistribute it and/or modify // it under the terms of the GNU Lesser General Public License as published by // the Free Software Foundation, either version 3 of the License, or // (at your option) any later version. // // This program is distributed in the hope that it will be useful, // but WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the // GNU Lesser General Public License for more details. // // You should have received a copy of the GNU Lesser General Public License // along with this program. If not, see <http://www.gnu.org/licenses/>. // ///////////////////////////////////////////////////////////////////////////////