What does the "readonly" flag do when creating an iSCSI LUN extent - 11.5.6 in the manual

[Garfje]

Hi
In the 11.2-U5 manual, Section 11.5.6, for creating a LUN extent mapping, the bottom of the dialogue says "Read-only". The helper icon text ways "Set to prevent the initiator from initializing the LUN".

Manual page ...

Screenshot of the Sharing / iSCSI / Extents / Add dialogue:

Can I ask what precisely this does? My thinking is ...

a) It stops FreeNAS from doing something to the underlying zvol when mapping the LUN to this extent? If so what?

OR

b) It makes the LUN extent read-only so it cannot be written to (weird to have this here)

Many thanks,
--R.