CARP/VRRP/etc. are using not only virtual IPs but also virtual MACs to make failover a smooth experience without clients or network equipment having to learn a new MAC address of a failover server like with only IP based configurations (early linux HA cluster for example).

The VHID setting is influencing which MAC is handed out for that CARP style VIP. All of them are (IMHO) using the failover MAC space of

00:00:5E:00:01:XX

so with changing the VHID you are also configuring the last "XX" segment of said MAC address. That's why it has to be unique on that network segment (L2) and you also have to watch out for other cluster/HA-grade setups, that are using VRRP or HSRP style VIP/MAC combinations. But if your pfSense cluster is the only cluster in that network segment, VHID 1 is commonly fine on all interfaces. We're using VHID 4 and 6 (for IP4 / IP6 VIPs on the same VLAN) over multiple VLANs just fine :)