PLT software is distributed under the GNU Lesser General Public License (LGPL). This means that you can link PLT software (such as MzScheme or MrEd) into proprietary applications, provided that you follow the specific rules stated in the LGPL. You can also modify PLT software; if you distribute a modified version, you must distribute it under the terms of the LGPL, which in particular means that you must release the source code for the modified PLT software.