Can someone explain the statutory immigrant preference system in the US?

An immigrant visa can be suppled to a spouse an employee, a relative.... pretty much anybody who fit the criteria

in the case of relatives ..some countries try to bring in more than the numbers of visa available so the wait can be longer for them that would be CHINA-mainland born INDIA MEXICO PHILIPPINES

but this does not effect Employment or marriage cases

Well, there are certain green card categories where there is no wait, such as for young children or spouses..

What you are talking about comes into play if a Permanent Resident is sponsoring someone, or a US Citizen is sponsoring a sibling or adult child who is married.

Yes, in those categories, no country can admit more than 7% of the total each year. As you can probally guess, countries like Mexico and the Philippines would easily make up more than that, so they are put on a waiting list. It is designed to make it fair for people who are not from countries with outrageously high levels of immigration.

In addition to the above answer, check out the visa bulletins issued by the State Department (link below). It will give you a bit of an idea as to which types of immigrants are subject to waiting lists and which categories must wait longer than others. If you can't find a particular category there, it means there is no numerical limitation.

It is important to note that only immediate relatives can be sponsored, i.e. spouses, children, parents and siblings and even among those, there are different categories. Also, legal permanent residents are limited in who they're able to sponsor (they can't sponsor married children, for example).

You can check the USCIS website, www.uscis.gov, to see the "preference" categories for sponsoring relatives if you're a U.S. citizen or green card holder. The two main factors behind the preference system are the status of the sponsor and the relationship of the sponsor to the would-be gren card holder.

Note that we admit far more green card holders under "family reunification" than we do under the employment-based visas--and that the only criteria for the former are that they be related to someone who's legally here and that that person show the ability to support them.