A more thorough explanation:

Definition: Modus decimandi is a Latin term used in Ecclesiastical law to refer to a special kind of tithing by custom that is different from the general law that usually required the tenth part of an annual increase.

For example, it could mean a twelfth part of a quantity of hay instead of a tenth part or a couple of hens instead of a normal tithing of eggs.

Examples:

• A farmer in a certain region may be required to give a twelfth part of his hay crop as a tithe instead of the usual tenth part.
• A church may accept a couple of hens as a tithe instead of the usual tithing of eggs.

These examples illustrate how modus decimandi refers to a specific custom of tithing that deviates from the general law of tithing.