Lossy Decomposition meaning Suppose I have a relation R=(A,B,C,D) and I decompose it into:

R1=(A,B,C)
R2=(A,D)

then if it is said that the decomposition is lossy,then what is the meaning of that statement?I have an understanding that the decomposition is lossy iff:

R1 ∪ R2=R

But can't see the meaning of this here because schemas are not same so we cannot take union.
As this is a loss less join thus in lossless join we know that on joining we can get original relation along with some extra tuples thus answere is r⊂S where r is the relation in which original reltions were decomposed.