Should I put a comma before the word "which" in the following sentence?
I have enclosed a copy of the assignment of the life insurance policy as collateral which outlines the information dated June 25, 1975 at the top of the page.
Probably yes, is the simple answer. If you want to know why, then read on.
I have enclosed a copy of the assignment of the life insurance policy as collateral, which outlines the information dated June 25, 1975 at the top of the page.
Without knowing the full context, it's impossible to be 100% certain, but the underlined relative clause appears to be of the non-restrictive (non-defining) kind, so it should be marked off with a comma. Restrictive (defining) relative clauses, the ones without commas, have a different interpretation. I'll explain how it works:
(1) Politicians who make extravagant promises aren't trusted.
(2) Politicians, who make extravagant promises, aren't trusted.
In (1) the underlined relative clause restricts the meaning of the noun "politicians". The lack of trust doesn't apply to all politicians, but just the ones who make extravagant promises. The information given in the relative clause is an integral part of the larger message: it plays an essential role in defining who is being said to lack public trust.
In (2), by contrast, the property of not being trusted applies to all politicians in general, not to a subset of them. Instead of picking out a particular group, i.e. the ones that make extravagant promises, like (1) does, it makes the claim that none of them are trusted.
Now look back at your example. The underlined relative clause is not implying that there is more than one assignment and it's only the one that outlines the information (and no other) that is enclosed. Instead, it appears to be saying that the assignment is enclosed, and that it outlines certain information. The fact that it outlines certain information is not necessary to identify which assignment is enclosed.