The Between Matching Brackets command is only available when bracket matching is enabled in the syntax coloring section of the file type configuration for the active file's file type. The syntax coloring scheme determines exactly which brackets are matched. See that section in this help file to learn how bracket matching works in EditPad Pro.
When you invoke the Between Matching Brackets command in the Block menu, EditPad Pro selects the text between the nearest pair of matching brackets that surround the text cursor. If some text was already selected, Between Matching Brackets expands the selection to touch the nearest pair of matching brackets surrounding the selection. The brackets are not included in the selection. If the selection already touches the nearest pair of matching brackets, then Between Matching Brackets expands the selection to include the brackets. Repeating the command expands it to touch the next nearest pair of matching brackets. You can repeat the command as long as there is a pair of brackets surrounding the selection.
Edit|Insert Matching Bracket
Go|Go to Matching Bracket
File Types|Colors and Syntax