该 - How do you know when to use it? Nothing to do with 应该
该 is the formal equivalent of the word aforementioned or said:


  该公司 - the said company
  该法案 - the aforementioned legislation


It is usually found in text to refer back to something that was mentioned previously.

Informally, you can also use 那. Here is the meaning from 汉典:


  那，著重指出前面說過的人或事物：～地。～書。