Selecting text says what to look for as well as what to copy. The other places the selected text occurs on the screen are painted in terminal_image<-isearch_other_style, the way the other matches of an incremental search are, so that picking out a variable or an atom finds the rest of them without typing it again; terminal_image<->exact_case and terminal_image<->search_word decide what counts as one. What it reports -- through visual->report, like a search -- says which of the matches the selection is and how many there are, as Selection: X (2/5), counted over the whole buffer while what is painted is the page.

Double clicking picks a word, and that says to look for the word rather than for the letters it happens to be made of: matching is whole-word for such a selection whatever terminal_image<->search_word stands at (see terminal_image<-match_word), so a double click on Bar passes over Barn while dragging over the same three characters does not. It also makes a one-character word worth looking for.

Not every selection is looked for, and terminal_image<-selection_string says which are. A selection that occurs nowhere else says nothing and is painted as a plain selection. Neither is one that is blank, that runs over a line break, that is longer than 100 characters, or that is a single character while terminal_image<-match_word is @off -- each of those would light up most of the screen. Nor is anything looked for on the alternate screen, whose lines are not in the buffer.

What is looked for is the text under the selection as it is now: a client that repaints or erases the screen changes what a selection holds without moving it, and the matches follow.