CSRK is related to LE security mode 2, data signing, see Vol 3, Part C, Section 10.2.2 in the Bluetooth Core 5 specification.
Some more information in Vol 3, Part H, Part 2.4.5:
An LE device can send signed data
without having to establish an
encrypted session with a peer device.
Data shall be signed using CSRK. A
device performing signature
verification must have received CSRK
from the signing device. The sending
device will use its CSRK to sign the
transmitted data.
LE security mode 2 is not support by any of our SoftDevices.
