It may be a word, phrase, sentence,or one or more paragraphs. Marking a block is most generally done in modern word processors by highlighting the block. Highlighting means that the text appears in inverse video on the monitor. The process of saving a file treats the entire document as the block to be saved. Other shortcuts may be available for END of document, END of line, TOP of page, etc.
Proportional fonts use individual widths for each character.
Affects the widths of lines.