x1=pagbu1=selci1 is an irreducible/fundamental/indivisible part or constituent or unit which partially composes whole x2=pagbu2.
x2 differs from selci2 in that x1 need not be the most basic form of a substance or entity and x2 may be a complex system involving many different such parts. For example, one may claim that organs (x1) are the most basic body-part (and they can work together in order to form an organ system, such as the cardiovascular or digestive system (x2)), but they are not the most basic unit of the body (which maybe is an organelle or DNA or something; selci2). Having multiple x1 will just make a large collection of that thing, not necessarily x2 (unlike with "selci": lots of water molecules make a collection of water but nothing more, lots of biological cells form an abstract mass of life but not necessarily anything more, etc.); x2 may have many x1s, each of which may be mutually very distinct. A gear, ink cartridge, needle, etc. could each be slepau of a printer (machine). Do not confuse with "selpau".