About the Sequence of Operations
COMSOL Multiphysics operates through sequencing and evaluates most of the branch nodes in the Model Builder from the top down as a sequence of operations. By adding nodes to a branch in the Model Builder in a certain order, you set up such sequences of operations, which makes it possible to, for example, parameterize a model and rerun the simulation. COMSOL Multiphysics then reevaluates each sequence, automatically updating the geometry, mesh, physics interfaces and features, and solution. A solver sequence, for example, could define your model with one solver and then, using the returned solution, solve it with an alternative solver.
For most sequences, you run the sequence by right-clicking the top node of the branch and selecting Build All (geometry) and Build (mesh), Compute  (studies), or Plot (plot groups), or by pressing F8. These buttons are also on the Settings window and in the respective toolbars.
Some nodes under a physics interface branch can override other nodes higher up in the sequence. How the COMSOL Multiphysics software treats those nodes depends on whether they are contributing or exclusive nodes (see Physics Exclusive and Contributing Node Types).
The sequence of operations means that the order of the nodes in the tree is important. In the following branches of the model tree, the node order makes a difference, and you can move nodes up and down to change the sequence of operations for these nodes: Geometry, Material, physics interfaces and features, Mesh, and Solver.
Also, the order can have some importance in the plot groups in the Results branch and also for the Perfectly Matched Layer and Infinite Element Domain nodes in the Definitions branch (those nodes are available with some of the add-on modules).
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Physics Node Status
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Physics Exclusive and Contributing Node Types
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Creating a Geometry for Analysis and Working with Geometry Sequences
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Moving Nodes in the Model Builder