Searching for the 'purchase options' data column to pull using admin REST API

In the reporting view on the sales over time, there is an option to include an extra column called ‘purchase option’. For our shopify, this brings a column with ‘subscription’ and ‘one-off’ which refers to the order being a subscription model or a one-off purchase. I would like to pull this data from the shopify API but I am unable to locate where this data is held. It does not seem to be related to sellingplan but I may be wrong.

photo of the reporting view with purchase options.