The Visa Bulletin provides valuable information on the immigrant visa numbers availability. The Visa Bulletin is a monthly publication by the Bureau of Consular Affairs, that is a part of the Department of State.
Section 201 of the Immigration and Nationality Act establishes limits on family and employment based immigration to the United States. The U.S. Congress determines the number of immigrants allowed to enter the U.S. for each category and country. There are no numerical limits in the Immediate Relative (IR) category that includes the parents, spouse and minor unmarried children of the U.S. citizens. However, other categories of the family based immigration has an annual limit of just under 500,000. The employment-based immigration has an annual limit of 140,000. Further, no more than 7 percent of the visas may be issued to natives of any one country. That means a tiny island country gets the same number of immigrant visa numbers such as huge and populous countries like India or China.
Visa Bulletin lists the availability of immigrant visa numbers during the month of publication. Visa bulletin serves as a guide for consular officers, attorneys and the applicants of the immigrant visa.
Visa Bulletin includes the statutory numbers for family sponsored categories and employment based categories. Visa bulletin also includes the statistics for diversity lottery approvals, refugee applications, as well as specific immigration application date from different countries. Visa Bulletin also has data on Diversity Immigrant Category cut off estimates.
Visa Bulletin essentially shows the backlogs in each immigrant visa category for a given month. The word "Current" (indicated by "C" in Visa Bulletin) indicates that currently there is no backlog in a particular category. The word "Unavailable" (indicated by "U" in Visa Bulletin) indicates that you can't apply for immigrant visa in that category.
Any date in any category in the Visa Bulletin indicates the "cut-off date" for a given immigrant visa category. If the priority date is earlier than the cut-off date, it means that the immigrant visa number is available and the beneficiary can file either for adjustment of status or go with consular processing. Otherwise, the person must wait until the cut-off date is past the priority date. If there is a cut-off date or it says "Unavailable" under the specific country/combination, that means it has exceeded their allocation of immigrant visas, they are "oversubscribed" and they must wait until their turn.
As all the countries get the same number of immigrant visa numbers, countries like India, China, Mexico and Philippines are more "Oversubscribed" than others. Therefore, the Visa Bulletin lists the cut-off dates for those four countries separately than rest of the world. If you were born in any other country, or due to cross-chargeablity (due to marriage to a citizen of other countries, for example), if you fall under any other country, you should look at "Worldwide" category that applies to rest of the world, except the listed countries.