As stated in their paper https://eprint.iacr.org/2019/099.pdf : "...the SRS is updatable, meaning an open and dynamic set of participants can contribute secret randomness to it indefinitely. Although this is still a trusted setup in some sense, it increases confidence in the security of the parameters as only one previous contributor must have destroyed their secret randomness in order for the SRS to be secure".
Basically, any party can update the Structured Reference String (SRS) at any point in time to increase the security of the protocol that will affect to any proof computed from then on.