Section 110.24 is new to the NEC as of the 2011 revision. Installers now must mark service equipment (other than residential) with the available fault current. The marking must include the date of the fault current calculation, and it must be durable.

In other requirements for marking, the NEC explicitly says legible. That isn't stated here. However, since it makes no sense to provide illegible markings, legibility is inherently required.

A good way to provide a durable, legible marking is to create a laminate material sign that you affix with screws. Such signs are often used to identify equipment, post warnings, and provide other information. So the means of making this sign are usually already onsite or readily available.

An advantage of this method is that you can easily change the information as needed. That's important, because the information must be kept up to date. If there's a modification to the electrical system and it could affect the fault current, the fault current must be recalculated.

If the fault current number changes, then the marking must also change. If the new fault number is higher than the old one, you can't just change the sign. You must ensure the service equipment ratings are sufficient for the maximum available fault current at the equipment line terminals. That process could be time-consuming.