In VMTL transformations, patterns are expressed as model fragments in the host modeling language. VMTL defines its own textual annotation language for such model fragments. VMTL annotations support pattern specification, model manipulation, and transformation execution control.

As a prerequisite for using VMTL, the host modeling language must support annotations that can be anchored to specific model elements. It must be possible to anchor an annotation to more than one model element, as well as leave it un-anchored.

VMTL annotations are declarative. They consist of one or more clauses connected by logic operators, where a clause is an assertion made about the annotated model elements or about its containing pattern/rule/transformation as a whole. Special variables are used for execution control, and user-defined dynamically-typed variables are used to pass values between clauses and between different patterns.

User-defined variables can be used in any VMTL pattern. They may appear wherever the host modeling language accepts a user-defined meta-attribute value, such as for representing the name of a model element. In addition, they can be manipulated via operators within VMTL annotations.

User-defined variables have global scope within a rule application: once declared, their value can be accessed in all patterns included in the applied rule. Their global scope makes variables suitable for identifying corresponding model elements located in different patterns.

VMTL defines a naming convention for user-defined variables: all variable names are prefixed by the $ character.

Apart from the Boolean, Integer, Real, and String types found in many general-purpose languages, VMTL defines the Element type used for variables holding host language model elements.

Regardless of their type, all VMTL variables accept the undefined value: *. When a variable has this value, it is interpreted as representing any valid value of that data type.

VMTL defines operators that can be used on both VMTL variables and model element properties. The arithmetic, comparison, and logic operators are similar to those found in many general-purpose languages. The . model navigation operator is used to access model element properties.

VMTL defines special variables as a method of controlling transformation execution (the injective, priority, and steps variables) and accessing the contents of the source model (the id, self, and type variables). From a syntactic perspective, special variables are treated just like regular variables.

Each special variable has a scope, identifying the fragment of the transformation specification to which the value of the variable is applicable. For instance, every VMTL rule has its own priority, which is independent from the priority of other rules. Therefore, the scope of the priority special variable is limited to one rule.

Clauses are the building blocks of VMTL annotations. Each annotation contains one or more clauses connected by logic operators. Clauses are assertions made about the pattern model elements to which they are anchored, about their containing pattern/rule/transformation as a whole, or about special or regular variables.

Some clauses dictate modifications to the source model (create, delete), while others act as constraints on valid matches (either, indirect, omit, optional). Variable assignment (:=) is also treated as a clause.

The either clause can only be included in annotations anchored to several pattern model elements. All other clauses can be included in annotations anchored to one or more pattern model elements. Anchoring a clause to several model elements promotes compactness of specification. Finally, the variable assignment clause (:=) can also appear un-anchorerd to any pattern model elements.