With the C2, Jolla proposed a subscription model to ensure continuous revenue. The issue to solve is a single payment of 25-50 € per new Sony device is very little money for Jolla to live on assuming one only needs a new device every ~3 years.
A number of people preferred one-time purchases, or yearly payments rather than monthly. Jolla heard and changed the subscription model to voluntary which as @dmz said basically makes it a donation.
Check the excellent answer to this question: “What happens when my subscription ends?”