3206188 - The conversion result between base unit and alternative unit is wrong for a material reservation

You have maintained a base unit Z1, which is used in reservations, and an alternative unit Z2, which is used in BOMs, for a material in the master data.
However, the value of the reservation in unit Z1 is not equal to the conversion result from unit Z2 based on the conversion relationship, but is rounded to a wrong one.

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SAP S/4HANA Cloud

The value of reservation is rounded because the rounding decimals of the base unit is not maintained properly.
For each Unit of Measure, the available decimals are not infinite. You have to maintain the number of decimal places to which this measurement unit should be rounded.
If the conversion result's decimal places is larger than the defined one, the rounding will be triggered. 
The rules of rounding vary in different scenarios. For reservations, to make the supply sufficient, the values will be rounded up.

To make the value of reservation accurate rather than rounded, you should maintain the proper rounding decimals for the base unit to make sure the decimal places of the conversion result is within the range.
You can do this configuration through SSCUI 102179 - Check Units of Measurement. Under the Conversion part in the details page of the unit, there is a "Decimal Rounding" field:

You should maintain a large enough value for this field if you don't want the rounding to be triggered.

base unit, alternative unit, reservation, conversion relationship, Decimal Rounding, SSCUI 102179 - Check Units of Measurement , KBA , PP-MRP , Material Requirements Planning , CA-GTF-CSC , Country/Region -Specific Application Interface Objects , Problem