A type "MOD 10" number part creates a check digit according to the Luhn-Algorithm, also known as "Modulo 10" or "mod 10" algorithm as well as the Double-add-Double-method.
The Repeatable element Selected part accepts references to other number parts, which are concatenated as input to the checksum calculation.
IMPORTANT:
The algorithm accepts decimal digits (0-9) exclusively. All other characters (letters or special characters) are omitted from the concatenation of values provided by Selected parts. Effectively, this makes the checksum calculation robust against non-numeric fill characters or separators. On the other hand dynamic number parts (see Dynamic (Number part)) should not specify base values > 10.
Examples of components:
Country: Fixed (Number part) with the value 400 (prefix for Germany).
Company: Resolve value (Number part), extracting a 4-digit company code from a text attribute in the data context.
Article: Dynamic (Number part), automatically incremented 5-digit article code.
Since the preview (right in picture) does not feature a data context for the Resolve value (Number part), the company code resolves as "0000".
NOTE: For obvious reasons it does not make sense to include the number part for the checksum as a Selected part.