When do I need a license to use Qt’s framework, tools, or libraries?

You have two options for your product team; a) Either you comply to the Qt Community Edition open source licensing terms and conditions, or b) you purchase a commercial developer license for every developer in the team. You cannot do both. 

The commercial Qt license gives you the full rights to create and distribute your software on your own terms without the open source license limitations or obligations. You will also get access to features, releases, and support that are not available for open source users.