Hi Anton, 

You can create the unit of measure in the same category, like the liter as the reference category and the drum as the bigger category, by mentioning the conversion ratio. and by default there is no option to add the vendor-wise reference for the unit of measure; by adding a field in the unit of measure, you can give identification reference for the vendors and create multiple units of measure with different conversion ratios.

And also you can map the unit of measure that you have created for the particular supplier from the vendor list under the product form.


https://www.odoo.com/documentation/19.0/applications/inventory_and_mrp/inventory/product_management/configure/uom.html