CFA 关于tax loss 和tax credit的问题

复习CFA时看到tax loss和tax credit，不是很明白这两个词的意思，另外在还看到有tax allowance，这三个分别是什么??

另外想问问这一句话的意思：As of 31 August 2006, the company had aggregate US tax net operating loss carryforwards of 1.7billion and unused US tax credit carryforwards of 164million. The company also has unused state tax net opearting loss carryforwards of 1.4billion and unused state tax credits of 163million. (in dollar)

这句话里的tax net operating loss carryforwards和tax credit carryforwards是什么，那tax credit/loss carrybacks呢？


还有为什么前面的unused US tax credit carryforwards is 164million然后后面又成了163，这两句话有什么不同么，求解释下，专业的名词如果可以的话麻烦举个通俗易懂的例子，我搞得好头晕，谢谢

Tax credit:
An amount of money that a taxpayer is able to subtract from the amount of tax that they owe to the government. The value of a tax credit depends on what the credit is being provided for, and certain types of tax credits are granted to individuals or businesses in specific locations, classifications or industries.

Investopedia上将Tax loss carryback, carryforward定义为：

A tax benefit that allows business losses to be used to reduce tax liability in previous and or following years.  

Investopedia上将Tax loss carryback, carryforward定义为：

A tax benefit that allows business losses to be used to reduce tax liability in previous and or following years.  

我不能确定，担心误导你，但是我在澳洲学习的，这东西就是你运行公司亏损了，把你亏损的部分乘以你的tax rate.this is the tax benefit, next time u need pay tax u can use this to deduct the tax amount u need to pay if u make a profit