All ELIMIT attributes are exact limits. Standard limit attributes operate by taking the index and converting it into an exact Lowrance Mercator Meter/Pixel limit using values on the graph on Page 28 on the IMC V0.7.4.0 documentation. The ELIMIT attributes work as a method to bypass that conversion and give a bit more granularity to the limit system. Most people that use that feature use it to match their vector data to raster data so vector/raster composite layers come off and on together. For traditional use, the standard limit indexes provide specific enough limits.
A Mercator Meter is meant to be a representation of 1 meter at the equator (in degrees). As you move North or South of the equator, this value becomes proportionally smaller based off the circumference of the Earth at the positional latitude versus the circumference of the Earth at the equator and is infinitely small at either of the poles. A Lowrance Mercator Meter, for reasons of projections and horizontal datums, is very slightly different from a traditional Mercator Meter and understanding why can be difficult for someone not familiar with different projections and horizontal datums.