Unused green cards can be moved between different categories but in a specific order. E5 has a maximum of 9,940 green cards that can be issued in any year. But what if there was demand for only 6000 green cards in the category in a particular year? The unused green cards can be used towards E1 so they are not wasted. Spillovers from E4 and E5 go to E1. Spillovers from E1 go to E2 and spillovers from E2 go to E3.

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There are great articles talking about spill over and another one.