The phrase "bottom tier" refers to something or someone being at the lowest or least important position or level among a group or a hierarchy. Full definition
The convenient bottom tier is perfect for storing books, magazines or remote controls, leaving the top tier open. (geekwrapped.com)
Silver status may be more valuable if you live in a city where bottom tier elites can score upgrades with high success rates. (rewardexpert.com)
Of the schools with the lowest graduation rates, none were also in bottom tier on student growth, and only six percent were in the group of schools with low college - going rates. (educationnext.org)