The relationship of table MSC_UNITS_OF_MEASURE and table INV_UNITS_OF_MEASURE_B

Summary:

We recently added an UOM from UI, then we can query it from table INV_UNITS_OF_MEASURE_B, but cannot query it from table MSC_UNITS_OF_MEASURE.
Previously we added the UOM from UI with the same steps, we can query from both of table MSC_UNITS_OF_MEASURE and table INV_UNITS_OF_MEASURE_B.

Content (please ensure you mask any confidential information):

<Question>
1, May you please let us know the relationship of table MSC_UNITS_OF_MEASURE and table INV_UNITS_OF_MEASURE_B?
2, In the mentioned scenario as above, can we think the UOM will be firstly stored in table INV_UNITS_OF_MEASURE_B? Then