What is the purpose of using movement accommodation factor for joint sealant?

Movement accommodation factor is commonly specified by manufacturers of joint sealants for designers to design the dimension of joints. It is defined as the total movement that a joint sealant can tolerate and is usually expressed as a percentage of the minimum design joint width. Failure to comply with this requirement results in overstressing the joint sealants.

For instance, if the expected movement to be accommodated by a certain movement joint is 4mm, the minimum design joint width can be calculated as 4÷30% = 13.3mm when the movement accommodation factor is 30%. If the calculated joint width is too large, designers can either select another brand of joint sealants with higher movement accommodation factor or to redesign the arrangement and locations of joints.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.