Correct me if I am wrong, but from the text excerpt "...NVS in-flash image will stay compatible unless the physical ATE size changes", my guess is that you're reading Zephyr documentation about Non-Volatile Storage. In this documentation the ATE is an abriviation for "Allocation Table Entry". In NVS, the flash area is divided into sectors, where some of the sectors are currently in use as storage, while others are ready to be used (empty).
From what I can understand, the ATE holds the information about what areas of the NVS that are ready for use or not. In the API Reference section in the linked document, the parameter "Allocation table entry write address" is explained as "Addresses are stored as uint32_t: high 2 bytes correspond to the sector, low 2 bytes are the offset in the sector".
