9.3 Data rules 257 9.3 DATA RULES Data rules (introduced in Section 4.9.2 in Chapter 4) constrain the data included in a transaction (a form or message) or a persistent data set (e.g., a database record). Each of the following subcategories of data rule has its own template(s): 1. data cardinality rules, which place restrictions on the maximum or minimum number of occur- rences of data items: a. mandatory data rules, which mandate the presence of data; the templates for these are described in Section 9.3.1.1; b. prohibited data rules, which mandate the absence of some data item in a particular situation; the template for these is described in Section 9.3.1.2; c. maximum cardinality rules, which place an upper limit (usually but not necessarily one) on how many instances of a particular data item there may be; the template for these is described in Section 9.3.1.3; d. multiple data rules, which mandate the presence of two or more instances of a particular data item in a particular situation; the template for these is described in Section 9.3.1.4; e. dependent cardinality rules, which mandate how many of a particular data item must be present based on other data item(s); the template for these is described in Section 9.3.1.5;