logical group of instances within single availability zone. Using placement groups enables application to participate in a low latency , 10 GBps network.
A placement group cant span over multiple availability zones.
Its a single point of failure.
Name of the placement must be unique within your AWS account.
Only certain types of instances can be launched in a placement group (C, G, R, I) Compute, GPU, memory optimized and Storage optimized.
AWS recommends homogeneous (Same size and same type) instances within the placement group
You cant merge placement groups.
You can moves an existing instance into a placement group. You can create AMI from existing instance and launched into placement group.