Placement groups are a setting that determines whether virtual machines will be hosted on the same physical server or on different ones.
Types of groups. There are affinity and anti-affinity ones. An affinity group assembles equipment on one hypervisor — the machines included in it will be launched on the same physical server. Anti-affinity works the opposite way: all machines in this group will be launched on different physical servers.
What it is used for. Each group has its own advantages. Machines launched in one affinity group will exchange data faster because they are located on the same server. Anti-affinity group increases fault tolerance of a cluster: even if something happens to one server, machines on the other will remain available.
What happens if you don't add instances to a placement group. They will be placed on physical servers randomly — may get on the same one or on different ones.
How many placement groups a machine can belong to at the same time. Only to one group — it is impossible to add it to two groups at once, even if these groups are of different types (affinity and anti-affinity).