Onshape provides the Create selection dialog to make selecting related faces, such as faces that define a pocket on a model, easy. This is especially useful in certain commands, such as Delete face and Replace face. An alternative to Create selection is the Select command through the same context menu:

Accessing Create selection from the part context menu

This command allows you to select a group of like items without having to open the Create selection dialog. You can use the collection of selected items to fill the tool dialog you open next.

For example, with one edge selected in the model below, you can select Select > Parallel edges to automatically select all the edges in parallel:

Create selection example, with one edge of the model selected

Create selection example, selecting parallel edges of the model

Then, when you open the Fillet tool dialog, the selection field is already populated with the selection:

Fillet dialog

To create a selection of connected edges, select an edge, then right-click to open the context menu:

Example of loop/chain connected edges option highlighted in a context menu

From there, click Select > Loop/chain connected edges.

Alternatively, you can use the Create selection command from the same context menu to open the Create selection dialog box:

Example of Loop/chain connected edges option used on a model in the graphics area