Using "if" with datediff to make conditional statements

[MB]

Hi, I'm having trouble understanding the following statements. They work but I don't understand the whole code.

These statements are used as data quality checks.

Future date entered: Date of consent

if(datediff([date_of_consent],"today","d","dmy",true)< 0,"999","111") = '999' and [date_of_consent] <>''

Date of test at the time of HIV diagnosis is before the HIV diagnosis date

if(datediff([date_of_test_hiv],[date_of_hiv_1_diag],"d","dmy",true)> 0,"999","111") = '999' and [date_of_test_hiv] <>'' and [date_of_hiv_1_diag]

What I don't understand is the " < 0, "999", "111" " part?

What does it mean with regards the whole equation?

Thanks!