UML 2 Frame Style Guideliness

A frame in the UML encapsulates a collection of collaborating instances or refers to another representation of such. Frames are depicted as rectangles with a notched descriptor box in the top left corner. Frames come in two flavors, a diagram frame such as Batch Transcript Printing and combined fragment frame such as the loop frame, both in Figure 1. Diagram frames explicitly define the boundary of a diagram whereas combined fragment frames encompass portions of a diagram or provide references to other diagrams or method definitions.

Figure 1. A frame encompassing a sequence diagram.

Figure 2. Modeling alternate courses of logic.

Figure 3. The internals of the seminar component.


  1. Avoid Diagram Frames. The Batch Transcript Printing diagram frame of Figure 1 adds a significant amount of visual clutter in the process.
  2. Use Interaction Occurrences Over Part Decompositions. There are two references to logic external to Figure 1: the TranscriptBatch object includes a reference to PrintRun and there is a combined fragment referencing the SharedServices.print() method. The style of the first reference is called a part decomposition and the second an interaction occurrence.
  3. Fully Specify Operation Names in References.
  4. Depict One Interface Per Port. Ports are connection points between a classifier and its environment which, are depicted on the side of frames as small rectangles.
  5. Depict One Port Per Realizing Class. See also UML interface style guidelines.
  6. Deemphasize Frame Borders. In Figure 1 you see that the frame border is lighter than the lines around it.
  7. Apply Standard Labels to Descriptors. Table 1 summarizes common labels for diagram frames and Table 2 the common labels for combined fragments.

Table 1. Diagram frame labels.

Label Usage
Component The frame depicts the internal design of a component.
Package The frame depicts the internal organization of a package, often using a UML class diagram or a UML use case diagram.
sd Indicates that the frame contains an interaction diagram, usually a UML sequence diagram although UML communication diagrams are also common options.
Use Case The frame depicts the logic of a use case, often as a UML activity diagram or a UML interaction overview diagram.

Table 2. Combined fragment labels.
Label Usage
alt Indicates several alternatives, only one of which will be taken, separated by dashed lines. Used to model if and switch statements. See the example in Figure 2.
assert Indicates that the fragment models an assertion.
criticalRegion Indicates that the fragment must be treated as atomic and cannot be interleaved with other event occurrences. Often used within a par frame (Douglass 2004).
loop Models logic which will be potentially repeated several times.
opt Models optional logic depending on the run-time evaluation of a guard.
par Indicates several fragments of logic, separated by dashed lines, all of which will run in parallel.
ref References another diagram or a method definition.