subset of attributes

[Alpha Beta <dzjitaru@xxxxxxxxx>]

Hi,

while reading about databases, I didn't understand what can be subset of attributes, see the following :

Let Ri (Xi) be a relation sheme, where Xi is a set of attributes. and if t is a tuple  for Ri (Xi) and Y is a subset of Xi, then t[Y] denote the subtuple of t corresponding to Y.

does a subset of attributes just means the instances that exist in each tuple?

Best,