I am using data separation model in order stock module and the total tax is coming from the item setting table but he calculation field is not coming.how we get calculation field in data separation model.

The separation model would have nothing to do with this.

The calcs will still occur in the data file tables generally, and based on the same relational ideas. Unless it is just for display in the interface.

calculation field do not work in dsm model.thanks for your answer but calculation firld of different table do not work

It's been a while, so you may have either moved on, or figured it out for yourself, but here is what I think the problem may be...

If your calculation depends on a related field, then you need to create the appropriate relationship in the data file, AND specify the context for the calculation.