楼主似乎问的是面板数据问题.

经济学中，前者应该是面板数据的一种表示方法，而后者是用来表示时间序列的。

i --- individuals

t --- time

Xit --- the variables for different individuals in different time.

Xt  --- the same observation in different time.

if you use {x,y,z,...} to define diffrenet individuals, then Xit = Xt.