Hi @Houzy Welcome to the community! I believe this is a CSS class that is needed, but @mathio can confirm this for me. 

Hi @Houzy and @Liz . Right now this query parameter is used only internally. However in future, this parameter might be used to provide deeper analysis about where your typeform responses come from.

It is safe to omit it in your typeform URL, however if you use our embed SDK (or embed snippet from share page) this parameter will be added automatically to the URL and you dont need to worry about it.