Obligatory precaution means that you must either act on it and apply it, or refer in the specific matter to the second highest Marja' of Taqleed who has a Fatwa (verdict) in the specific matter. When the Marja' of Taqleed has not reached to firm evidence to be able to issue a definite Fatwa (verdict) in any matter, he states that it is a precaution to do or not to do.
Precaution is either obligatory when it is mentioned without mentioning any verdict before or after it, or recommended precaution when it is mentioned after or before a verdict.

If the Marja' write: It is permissible to do so but it is precaution to leave it. This means that it is a recommended precaution because he has mentioned that it is permissible to do so. If he states: It is a precaution not to do so. This means that this precaution is obligatory which gives you one of two options; either to apply the precaution, or to refer to the second highest Marj' in knowledge who has a verdict in the specific matter.

Wassalam.