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Most of the avionics I see have a label that says "Mods" and then various numbers checked 1-15.

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Image source.

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.

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  • $\begingroup$ Pretty much. I think some equipment may not have all of the mods. Like having mods 1, 2, and 4 if mod 3 wasn't required for whatever reason, but that can be a big headache to manage from a unit configuration standpoint $\endgroup$ Sep 27 '21 at 19:19
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    $\begingroup$ Each manufacturer will do it differently as well. Often the mod state is a letter suffix after the Part No or Serial No. $\endgroup$
    – John K
    Sep 27 '21 at 22:14
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    $\begingroup$ "Mod" comes from modification. it is also common to refer to different mod levels as revs, short for "revision level" as in rev 1 (as issued), rev 2, rev 3, rev 4 as more revisions are incorporated into the original design. $\endgroup$ Sep 28 '21 at 4:14

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