There are four basic types of graphical objects in APN: places, transitions, tokens, and triggers. As the objects are created, they are asigned unique integer labels (starting with zero). When the model is saved, the labels are reordered, so that places are labeled first, followed by transitions, triggers, and tokens.
Joints are more complex objects and meaningful models can be created without them. They have separate enumeration.
Tokens have additional attribute called ID that are also assigned during simulation, and facilitate merging tokens (see Joints). Several tokens can have the same ID (when the duplicate tokens are created).
Indeces are assigned during the simulation and those indices (that also start with zero) are specific to each type of objects. In general if the model is not changed, the relative order of labels and indices is the same.
Labels and indices can change automatically as the model changes, so they are not assigned manually. Please use the names of objects to provide persistent, manually assigned identifiers.
When models are merged by using "Add from file" option from file menu, the model from a file is appended to the existing model, so if the model has N objects, and new labels of objects appended are shifted by N (label 0 becomes N, 1 becomes N+1, etc) while the labels from the existing model don't change until we save the resulting model (then the objects will be rearranged by type). Note that add from file objects are read into a new page(s), so both pages and subgroups are shifted as well.
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