This is good to see formalized:
Eligible Individuals (Program Director/Principal Investigator)
K99 applicants must have no more than 4 years of postdoctoral research experience at the time of the initial or the subsequent resubmission or revision application, and must be in mentored, postdoctoral training positions to be eligible to apply to the K99/R00 program. If an applicant achieves independence (i.e., any faculty or non-mentored research position) before a K99 award is made, neither the K99 award, nor the R00 award, will be issued.
Parental leave or other well-justified leave for pressing personal or family situations of generally less than 12 months duration (e.g., family care responsibilities, disability or illness, active military duty) is not included in the 4-year eligibility limit. In addition, time spent conducting postgraduate clinical training that does not involve research is not considered as part of the 4-year research training eligibility limit. Only time dedicated to research activities would count toward the 4-year limit.
Additional clarifications are provided under Frequently Asked Questions. Potential candidates are encouraged to discuss their individual situation with an NIH Institute or Center Scientific Program Contact before applying.
I hope this will help some postdocs with the conversations they have with their POs on this.