It should be possible to adjust the chair in different ways

In an ergonomic chair there should be enough adjustments to get the good sitting posture.

Height of the seat

Most important adjustment is the seat height. The height is right when Your feet are steady on the floor and Your knee angle is slightly more than 90 degree. 

Depth of the seat

You should be able to adjust the depth of the seat. Seat depth is right when Your wrist fits between Your knee and front edge of the seat. Then You have space enough to move Your feet and the front of the seat doesn´t press Your vains in your calves.

 Tilting mechanism and seat angle

To get variation and movement into Your working day, it would be good to have a tilting mechanism in Your chair. You should ne able to adjust the tilt tension and also to get the tilt tension right for Your body weight. Normally when You work with a standard office desk,You often lean forward and that is when your bloodcirkulation doesn't function well. It is then important to tilt the chair to get the bolldcirkulation running in your thighs. 

 Height of the Backrest

There should be a possibility to adjust the height of the backrest. It is important to adjust the lumbar support on the same height as Your S -curve of Your lover back. 

 Is armrests needed?

It depends of the desk. If Your desk gives Your arms enough support armrests are not necessary. But very often You need some support when You are doing "mental work", speaking on the phone or simple when You sit down or when You get up from the chair. The most important fact is that the armrests are easily adjustable.