If your website has sub-domains, each sub-domain requires a different Reactful code (see here how to add multiple websites).
However, you can connect these domains by using Cross Domain Goals.
This means you can create reactions on one domain, but track goal complete events on a different domain (currently only available with Page Visit Goals).
Cross Domain Goal Setup -
From the Dashboard or Studio, click "Create New Goal".
Select the Page Visit Goal. By default, you will see pages from the domain you are currently editing.
From the drop-down menu on the right, select the website the page goal is located on.
The list of pages will update to show the pages from the goal domain:
Select a page from the list, or create a custom url / path.
Click continue, name your goal and save it.
Due to browser restrictions, cross-domain goals cannot be tracked for Safari users.
The "Data Flowing" indication in the dashboard will only appear once a visitor reaches the goal page after visiting the original domain.