Pakistan does not have the caste system. It is a "biradri" system which is often termed as zaat or caste system but it is nowhere close to the segregational hindu caste system, although it is still very inhumane.
I will take a stab at describing our socia-economic "biradri" system hoping to clear any confusion within both Pakisani and Indian PPers.
The typical caste system in india is based on four or five major groups such as Brahmins, Kshatriyas, etc and their standing in society, professions, etc are driven by their castes.
The biradri system in Pakistan is based around religious/tribal/linguistic/past professional affiliations. For instance, some will tell you they are pathans by zaat, when what they mean is they are pathans by ethnicity or biradri, and its not exactly the sort of caste system that hindus practice.
The closest similarity in the pakistani "biradri" system to the indian caste system is one where a particular group associates themselves with a tribe dealing with a particular professions e.g sunyarey, kumhar, etc or Gujjars who are commonly associated with the dairy business or memons who are just considered businessmen. But in todays Pakistani society, Gujjars have other non dairy related careers and memons are also going into non business related careers.
However, that does not mean all is well, we do have our issues too. For instance, the minorities in Punjab are stuck doing menial jobs and now it has become a norm that all the janitorial or cleaning and trash work has to be done by low level Christians. There are educated Christians too who work as professors, judges, actors, singers, etc but this is sort of a stigma in PAKISTAN.
Hindus in Sindh typically engages in commerce and do pretty well contrary to what some Indian hindus here think. The Pakistani hindu families are well established in textile business and they try to keep it hush hush and under the radar.
So you see our segregation is along the lines of anything you can think of under the sun, but the society rarely imposes any restrictions on them in regards to what they can do. The most prominent use of our biradri system is the use it offers when families look for marrying off their kids because they tend to prefer to stay within their biradri.
I thought its important for people here to understand the delineation of the two carelessly thrown around "caste" concepts either side of the border here.