If you comply with the
AGPL-license , you can use the Vaadin Timeline for free - even in commercial projects. Most commercial projects do no comply with AGPL as one of its requirements is that the project source code must be released with a (compatible) Open Source license and all recipients have right to re-distribute and modify it. Please see the
GNU FAQ for more info.
If you do not comply with AGPL-license, but would like to use Timeline - you must obtain a commercial license. In that case you can use the addon in your commercial application without disclosing source code or giving your customers re-distribution rights. Price is per developer: you are free to sell/distribute any number of copies of your software (that contains timeline addon) without any extra payments per sold software license.