The tkscript source code is distributed under terms of the GNU General Public License, see GPL for legal details.
 
This does not mean that you may not use TkScript in commercial products but if you do:
Either

• Contribute your changes back to me (see Authors)
• Use it as-is without changing the source
• ..or simply do not re-distribute it outside your department/group

 
Last but not least, applications written in TkScript do not necessarily have to be licensed under terms of the GPL, as well ! (see Your code).
 

The YAC plugin interface headers and dummy-host implementation are distributed under terms of the GNU Lesser(Library) General Public License, see LGPL for legal details.

Plugins may be placed under whatever license the author sees fit.
 
Most plugins available on this site use the LGPL license, some use the MIT license. See the respective plugin documentation for details.

Since it's your code you're free to decide on the software license for your own script applications and libraries as long as you respect the licenses of any 3rd party libraries/code you used.

In case your application uses any script library (.tsl), you'll have to comply with the license that the respective script library uses.
 
Using a script library is considered linking, not a derivative work.
 
Please see the LICENSE file (or documentation/source) that came with the respective script library for legal details.
 

You should use the lib: syntax in your project file (.tkp) so that the library is not included in the .tkx binary application file but rather loaded from the usual TKS library directories (e.g. /usr/lib/tks/libraries or the current working directory).

The tkui and tkunit script libraries that are available on this site (www.tkscript.de) are distributed under the terms of the GNU Lesser(Library) General Public License (LGPL).
 
This means that you can license your application however you wish, as long as you a) use these LGPL script libraries as-is or b) make your changes to those libraries publically available.