Naming convention for instance names

Summary:

What are the naming conventions for instance creation? We tried to create a custom suffix and it did not allow

Content (please ensure you mask any confidential information):

We know from the screen, that instance name should start with lowercase and less than 15 characters and no special characters and spaces in between.

custom suffix like -test, -dev are allowed.

Here is the case I have encountered error.

abcplan production type
abcplan-test test type
abcplan-dev production type (failed). so we tried abcplandev —-it worked with that.

Now if we try abcplan-stage with Test type it is failing.

Did any one try custom suffixes rather than test and dev phrases?