Including unknows as an "allowance" gives the future home owner a realistic figure to take to the bank to get a construction loan. If you leave out things that aren't decided yet, like fixtures, the bank won't know to add the expected price into the cost of the whole house.
Typically the contract reads that an "allowance" of $XX has been included for fixtures and then when the actual costs are determined later, they pay for fixture installation on T&M but they're predicting it will be around $5K.
When it's all said and done, either they'll owe you a little if they spent more than 5K of fixtures (including labor) or you'll get a little less than the contract price if it was under 5K.
