By Andy Kirk means, do the (viewers') need engage with this work, or is it entirely voluntary

This question seems to be incomplete. However, there is enough information to find the right answer.

Dynamic of need

Explanation:

The question refers to "Data Visualisation: A Handbook for Data Driven Design," by Andy Kirk.

Kirk lists some of the factors that influence the delivery of accessible design, such as

The dynamic of need refers to whether the audience needs to engage whit the work as a means to an end, such as a requirement for a job, or do they do it voluntarily.

The  Subject-matter appeal refers to the interest the audience has in the subject at the beginning of the consumption experience. Only those that find the subject matter immediately appealing will more likely engage with the data visualization.

Subject-matter knowledge refers to what the audience already knows about the subject and the potential to learn more about it.

the density would be a smaller number.