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Re: [dxwg] Profiles may provide rules on cardinality of terms (including “recommended”) [ID41] (5.41)

From: kcoyle via GitHub <sysbot+gh@w3.org>
Date: Sat, 29 Sep 2018 06:38:28 +0000
To: public-dxwg-wg@w3.org
Message-ID: <issue_comment.created-425620946-1538203107-sysbot+gh@w3.org>
Some possible text:

Profiles MAY include cardinality constraints for entities and elements. _(Using these terms for now; we'll need to be consistent across the document.)_ The primary constraints that are used have the following meanings:

- Required: this entity or element MUST be present in the metadata unit _(another term; do we use "record"?)_
- Optional: this entity or element MAY be present in the metadata unit, but it is not an error if it is not included.
- Repeatable: the entity or element MAY be repeated.
- Not repeatable: only one instance of the entity or element is allowed.
- Recommended: an entity or element that is not formally required but is considered to be an important element to provide if the metadata creator can do so.
- Mandatory if applicable: this is a lesser-used constraint that is relevant to some metadata communities whose descriptive area has variants that cannot be expressed with other constraints. 
- Minimum occurrence: this is the minimum number of times that the entity or element MUST be present.
- Maximum occurrence: the number of times that the entity or element is present MUST NOT exceed this number. 

In data modeling and programming languages, constraints are frequently defined numerically with a two-digit expression. The first digit represents the minimum cardinality and the second the maximum. This expression is interpreted to include mandatory (minimum cardinality is greater than zero) and repeatable (maximum cardinality is greater than one). There is no such formal definition in for recommended or mandatory if optional.

_(Can we make this SHOULD and give a reason?)_
_(There is the question of allowed elements with no specific constraints. These can be included in SHACL and ShEx as valid terms with no cardinality.)_

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