How are hardware tiers related to AWS EC2 instance type?

If you're running Domino on AWS machines, then each hardware tier is based on an underlying AWS instance type with some additional options that your Domino admins can configure

Can I have multiple hardware tiers per instance type?

Yes, you can have multiple hardware tiers per instance type (put another way, there can be more than one tier built off a certain instance type). For example if you could have a 'Shared' tier that is based on m5.xlarge and allows 4 different runs or jobs to share the same machine, and you could have another 'Dedicated' tier that is based on m5.xlarge where only one run or job can use the machine at a time such that it has the full resources of that machine available to it at all times. (Each of these hardware tiers can have multiple executors going at once, so for example you can have 8 jobs sharing 2 m5.xlarge instances on the 'Shared' tier and 3 jobs each using their own machine on the 'Dedicated' tier for a total of 5 machines. The jobs could be coming from multiple users or all from one user. ) Each hardware tier definition should say what ec2 type it is in one of the fields, so if you're an admin on your Domino you can check what instance type underlies each tier.

What happens if I need to spin up a machine and there is room for more of that specific hardware tier, but another hardware tier of the same EC2 instance type has our account at the limit for that instance type? Can I manually create the one that I need?

If you have multiple tiers built on the same instance type, call them tiers A and B, and you hit your AWS instance limit by launching machines on A, then you will get an error when you try to launch machines in tier B letting you know that you have hit an AWS instance limit.

Does the machine limit in Domino reflect the limit in the respective AWS account configuration?

The Domino instance limits can be totally different from the AWS instance limits for their AWS account, as the Domino limits are set separately from the AWS limits. So for example, if you have an AWS instance limit of 100 for the m5.xlarge instance type, you could have whatever instance limits you want for the Domino hardware tiers built on it, and you would stop being able to launch machines when you hit the lowest limit -- so if you set the Domino limit for a tier built on m5.xlarge to 50, then you could only launch 50 machines of that tier in Domino, and if you set the hardware tier limit in Domino to 150 you would get an error once you have launched 100 machines letting you know you've hit your AWS instance limit.