What different clay bar with pad? 
first of all, let us know to study clay block, the clay block have nice perforamnce for clay removing process.  the clay pad have a lay material is velcro in the back of clay block; 
1) a lay of clay on the sponge; 
2) sponge between of clay and velcro. 
3) velcro in the back of pad;  

the 3 layers are different funcation; 
clay for remove contaminants in safety
sponge supply water and reduce the polisher power. 
the velcro let the pad can work with polisher.  

so, the clay bar must be all materials in high quality. 
quality clay pad let the removing works in effictive.