Ok, sorry. I have the answer :
Signed system volume
macOS Big Sur introduces a cryptographically signed system volume to protect against malicious tampering, which replaces the separate system volume introduced in macOS 10.15.
For IT admins, this new volume makes for a very large macOS Installer file that native macOS tools or Jamf Composer can’t build as a .pkg file, which is the most common method to deliver macOS applications.
If you have Jamf or are using Jamf Composer to package the macOS Big Sur Installer to deliver via a policy, choose the option to “Create as DMG” instead of a .pkg file.
macOS Big Sur’s signed system volume (SSV) raises protection of key system files beyond the reach of all current malware and should withstand the most determined attacker from altering them after the OS has booted. It also guards against inadvertent corruption and guarantees system integrity.