Most of the avionics I see have a label that says "Mods" and then various numbers checked 1-15.
What does this information relate to?
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This indicates the "mod level" of the hardware in the unit. When a piece of equipment is designed and then certified, it is likely that none of the boxes will be marked.
Then say the product design has a hardware change made to it. Whatever that change consists of is then incorporated into new production units at "mod level" 1. Any units still out in the field won't have this change unless they are reworked to include the modifications. Once the rework is done, the mod plate is then updated to mark the "1" box.
Some installations may require all versions of that equipment to be at a certain mod level in order to meet the certification requirements of that installation.