An important objective of the three-tier architecture is to provide data independence, which means that the upper levels are unaffected by changes in the lower levels. There are two kinds of data independence: logical and physical.
Logical Data Independence
Logical data independence can be defined as the immunity of the external schemas to changes in the conceptual schema.
Physical data independence
Physical data independence can be defined as the immunity of the conceptual schema to changes in the internal schema.
