First start with defining the Unit of Measure.
For example, one class might be "Volume". How does your business measure "Volume" ? For example, the concrete business measures their product in cubic yards. Utilities such as a water department measures their product in gallons. These are aspects of the class known as "Volume". Another class might be "Distance". How does your business measure "Distance" ?
For example, the concrete business measures distance in miles (number of miles dispatch makes a delivery to the customer). A water department may measure distance in feet or even inches (amount of pipe laid to each customer. These are all aspects of defining the Unit of Measure Classes, which is understanding the needs of the business. Most businesses need many difference classes set up, for measures such as distance, volume, time, weight, etc. Some may not be applicable.
The base unit of measure is normally the smallest possible unit of measure. If the concrete company rounds off their miles, regardless and never is concerned about 1/10th of a mile, then 'mile' would be their base unit for distance.
Second define your Units of Measure. A primary unit of measure must be selected as the stocking unit of measure for an item in the organization.
Third define your Unit of Measure Conversions. Oracle does not know that 24 cans of soda make up a case. This is an intra-class conversion - like items. Suppose a can is the smallest unit of measure in this situation. 24 case equal a case. 12 cases equal a box. 8 boxes equal a pallet. 32 pallets equal a truck load. These are the types measures that need the to be set up for intra-class conversions for all of your items. If all of your items are cans, then this needs to be set up once. If you have many different dissimiliar items, then intra-class conversions need to be set up for each different type of item.
Interclass conversions. Lets suppose that you measure your soda cans in ounces, 12 ounces to a can. But your business also ships globally where the measure is performed in milliliters. Therefore a interclass conversion needs to be set up that denotes that one fluid ounce equals 29.583 milliliters of product.
If you are in the concrete business, an you normally measure your product in cubic yards. However many of your customers need to know how many cubic yards of concrete are necessary to cover 500 square feet (with an industry standard of 4 inch thicknesss). This is an Intraclass conversion - you are converting cubic yards into square feet of product.