As explained in many suttas, for example in MN121, in the gradual training approach the signless concentration is the next step after "neither perception nor nonperception" and the last step before The Final Liberating Realization.
If you search this site for the words signlessness, signless, nimitta, animitta, and semiotics - you will find many of my detailed answers on these topics.
For a detailed analysis of Pali sources, see this nice article: https://journals.ub.uni-heidelberg.de/index"Signless" Meditations in Pali Buddhism.php/jiabs/article/download/8674/2581/
This topic is of the utmost importance because it connects the Early Buddhism and Mahayana into one teaching.