To mark out an area or shape means to show where it begins and ends. When planting seedlings I prefer to mark out the rows in advance
If a particular quality or feature marks someone or something out, it makes them obviously different from other people or things. There were several things about that evening that marked it out as very unusual Her independence of spirit marked her out from her male fellow officers
[ 'märk ] (noun.) before 12th century. Middle English, from Old English mearc boundary, march, sign; akin to Old High German marha boundary, Latin margo.