Two types of placement groups:

• Clustered Placement Group
• Spread Placement Group

A cluster placement group is a grouping of instances within a single Availability Zone.

Placement groups are recommended for applications that need low network latency, high network throughput, or both.

Only certain instances can be launched into a Clustered Placement Group.

A spread placement group is a group of instances that are each placed on distinct underlying hardware.

Spread Placement Group are recommended for applications that have a small number of critical instances that should be kept separate from each other.

A cluster placement group can't span multiple Availability Zones. A spread placement group can.

The name you specify for a placement group must be unique within your AWS account.

Only certain types of instances can be launched in a placement group (Compute Optimized, GPU, Memory Optimized, Storage Optimized)

AWS recommends homogenous instances within placement groups.

You can't merge placement groups.

You can't move an existing instance into a placement group. You can create an AMI from your existing instance, and then launch a new instance from the AMI into a placement group.