2 questions on using Uniswap

Many coins listed on DEXes have 'tax' to buy/sell said coin. 

Ex. Buy 1000 COIN but receive only 950 COIN with 50 COIN as tax, many times this goes to fund their devs/marketing whatever.  This is in addition to the ETH fee when executing transaction.

1. I'm assuming user can declare whichever they choose as the 'Fee' and 'Fee Currency' when importing the trade (COIN or ETH).  Or is there clarification on this?

2. How exactly are fees figured into the overall accounting?