Deterministic Functions

I want to understand exactly how deterministic functions work and when I can use them?

For example I know they can be used in below example.

I have a table that data wont change. i.e.

office_company(office number(6), company number(6))

values

office                            company

1                                    100

2                                    100

3                                     211

In this structure I use a deterministic function that gets the office and returns the company and improves my query speed a lot. I know that it is safe to use deterministic in such data structure.

Consider below example

I have a table with employees and travel dates. These dates can change during the day. I have a function that gets the code of the employee and returns the travel date.