Flash recovery area would be shared by all databases from that oracle home and each database would then write to the location

/usr/app/oracle/flash_recovery_area/{SIDNAME}/archivelog/

if you have many databases like SIDNAME1 SIDNAME2 ...you will have directories like

/usr/app/oracle/flash_recovery_area/{SIDNAME1}/archivelog/
/usr/app/oracle/flash_recovery_area/{SIDNAME2}/archivelog/

its just a logical way of organising things by oracle no ?