Form-based creation enables users to construct a postcoordinated expression from the selections permitted by a dedicated form. This approach is useful for preventing errors when generating postcoordinated expressions, and is, therefore, applicable to a range of users, including:

• Non-technical users, or users with limited knowledge of the SNOMED CT concept model
• Expert users with very detailed knowledge of SNOMED but where the expressions to be built or the rules that constrain them are complex and so errors are easy to make

Form-based authoring may involve the use of a generic SNOMED CT expression builder, (see ) , or the form may populate a predefined expression template (see 4.1.3.2.3 Expression Templates).

A well-designed expression builder utilizes the features of the compositional grammar, the Machine Readable Concept Model, and the SNOMED CT Editorial Guide.  

Key features of an expression builder include:

• Enable the selection of the focus concepts to be wither refined or qualified
  • Either via the selection from a pre-defined set of options or from a less constrained search across the hierarchies
• Enable the addition of one or more refinements
  • The attributes available should be constrained to only those attributes that apply to the selected domain
  • The attribute values available for selection should be limited to the concepts within the permitted range determined by the selected attribute and focus concept
• Enable the addition of attribute groups, when these are required to represent the desired semantics

Example of an expression builder which supports implementers in constructing valid SNOMED CT postcoordinated expressions.

When using a form to create postcoordinated expressions it is the responsibility of the developers of the form to ensure that the generated expressions are valid considering the syntax and concept model rules. Users of the form should, however, be aware of the editorial guidance that apply to the domain of the expression.