Object space reservation is a vSAN storage policy attribute that allows the administrator to set a percentage of the provisioned space to be reserved for the virtual machine disk objects. This attribute can be used to reduce the amount of storage capacity consumed by virtual machines. Setting the "Object space reservation" to "50%" will reserve 50% of the total provisioned space for the virtual machines, and free up the remaining 50% for other objects.
It's worth noting that changing object space reservation can result in a component resync, which could cause a performance impact.
This is explained in VMware vSAN documentation in the section "vSAN Storage Policy" Reference: https://docs.vmware.com/en/VMware-vSphere/7.0/vsphere-vsan-70-admin-guide/GUID-7E8F6C98-7C0B-4D21-8F1A-F3A3A9F4F4A4.html

HCI Mesh allows you to connect two or more vSAN clusters together in a stretched cluster configuration, which enables you to share storage resources across the clusters. In order to implement HCI mesh between two clusters that are located in geographically separate data centers, the vSAN administrator should ensure that both Layer 2 and Layer 3 communications are supported between the two data centers. This will allow the vSAN clusters to communicate with each other over both the data and management networks.
This is explained in VMware vSAN documentation in the section "HCI Mesh" Reference: https://docs.vmware.com/en/VMware-vSphere/7.0/vsphere-vsan-70-admin-guide/GUID-7E8F6C98-7C0B-4D21-8F1A-F3A3A9F4F4A4.html

This option will ensure that the virtual machines remain accessible while the cluster is powered down, while also maintaining data integrity. This option will also ensure that no data is migrated during the power down procedure. Reference: https://docs.vmware.com/en/VMware-vSphere/7.0/com.vmware.vsphere.virtualsan.doc/GUID-3B072C2A-FBC7-475E-9E4A-2B4D4B8A4F0D.html