In Odoo, the default unit of measure (UoM) and the purchase unit of measure must belong to the same UoM category to ensure proper conversions. If they are from different categories, Odoo will prevent the user from saving the product.
For example:
* If the default UoM is "Units", the purchase UoM can be "Dozens" (since both belong to the "Unit" category).
* But if the default UoM is "Kilograms", you cannot set the purchase UoM as "Liters", because they belong to different categories.