Process selected items according to Options.
Selection may be in a file list or in a folder tree, depending on where the focus is.

Open with
Let you choose a program.

Send to
Let you choose a program from your Windows "Send to" context menu.

Program
Run the program with selected items. TE will append the paths of selected items to the command line behind the scene.

Open / Open in new tab / Open in background
Open selected items. See Type Open.
If any items cannot be opened, TE will process them in the "Exec" way below.

Exec

If you select multiple items, they must be the same type.

Any other things
TE will process them as Type Exec. This will probably not be related to selected items anyway.

You can use environment variables here.
See: Process selected items in view sequence