Q: Status of submission changed in a day
What does it mean if the manuscript's status changes from "awaiting reviewer selection" to "awaiting AE decision" after one day? Does it mean it is a rejection?
The review process varies from journal to journal. In your case, it seems that the manuscript has not gone through a peer review. It could be that the journal wasn’t able to find suitable peer reviewers for your paper, and the handling editor is evaluating the manuscript to decide on a suitable way forward. If this is the case, then the handling editor might refer the manuscript to another editor/editorial board of the journal for a final decision regarding the submission.
Since the change was made in one day, it's more likely that the change was made in error. If the manuscript status is stuck at “awaiting AE decision” for, say, two weeks, you could write to them requesting for clarification about the status change or simply an update.
You may also find these resources helpful:
Good luck and we hope you receive a favorable response!