There is no way to modify user macroses on host level for hosts, created from a some LLD host prototype rule, they are available for adding or modifying only at host prototype configuration in LLD.

In cases, when we would to use custom values for different host prototypes  (for example CPU or Memory usage for different VMs or Hypervisors)  - we cannot set custom values for thresholds and losing flexibility of monitoring

Example

{RB Template VM VMware Hypervisor:vmware.hv.memory.used.percents[{$URL},{HOST.HOST}].min(#2)}>{$MAX_MEMORY_PUSED}

Macros {$MAX_MEMORY_PUSED} will have one value for all host prototypes