The detailed specifications for dark fibre are usually negotiated between the owner of the fibre and the company leasing the fibre.
There may be different types of fibre available:
G652 of varying grades and vintages
G.656 TeraLight etc.
It is important if you are leasing fibre that you understand the details of the specifications and the impact that they might have on the operation of the systems that you might use to light up the fibre. Remember that some dark fibre may have been in the ground a long time and so the performance may not be the same as for new fibre.
You can learn about infrastructure specifications on OTT's Certified Optical Network Associate (CONA) programme. OTT can also provide expert independent advice on specifications on a consultancy basis is required.
But whatever the agreed specifications it is always best to make sure that the dark fibre is thoroughly tested to make sure it meets those specifications