Estimates need to reflect reality. Some basic characteristics of what we need from estimates include:
The estimates consistently reflect the capacity of the Team to deliver.
The estimates consistently reflect the size of the work no matter what kind of work it is. For example, it should not matter that we are estimating a new feature or defects.
They are truly relative in that, on a Team by Team basis, an 8 is an 8, and an 8 is about 4 times a 2 no matter the source.
Estimation data allows the use of ranges of estimates to help understand the risk profile of the work. A simple example of this is when the Product Owner uses best, average and worst velocities to understand what is likely to happen with their plan.
It is worth saying again:
The estimates consistently reflect the capacity of the Team to deliver.
They must be based on reality.