It depends entirely on what the agreement is between HMRC and your employer. (I'm assuming UK HMRC as your tax authority here).
In some cases it's tax free, in some cases part taxed, in some cases entirely taxed. It depends on what case your employer has made to HMRC about the payment being justifiable / required.
If you're tax free - you're laughing. If you're taxed in full then you can keep receipts for what you've spent and offset thos in full against your tax return.
It gets difficult if part of the p/d is tax free however because then you can only claim back the expenses above the amount that is tax free if you see what I'm getting at - this requires some arithmetic.
If subsisting outside the UK then you don't use reciepts - just use the HMRC figures for specific cities "HMRC Worldwide Subsistence Rates".
Hope that helps