While selecting or assigning a coiled tubing string to perform a job on a particular well, coiled tubing engineer makes sure that the CT sring can meet:

Withstand the maximum tensile load, when it is at the bottom of well. In addition it must be able to withstand a certain over pull in case BHA gets stuck in the wellbore.

Can achieve the maximum reach required in case of horizontal wells.

If it reached to target depth in the horizontal well, it must be able to transfer a certain weight to the bit(WOB) especially when we performing milling jobs with CT.

Allow the maximum required pumping rate within allowalbe pumping pressure.

Burst and collapse pressure limits are with in working pressure i.e. it must be able to withstand the maximum internal and external pressure which we have to encounter during job.