The Autism Classification System of Functioning: Social Communication (ACSF) describes social communication functioning levels. First developed for preschoolers with ASD, this study tests an expanded age range (2-to-18 years). The ACFS rates the child's typical and best (i.e., capacity) performance. Qualitative methods tested parent and clinician perspectives of the ACSF age expansion using content analysis. The ACSF was used twice by parents and professionals for the same child/youth. Reliabilities were assessed using weighted kappa. Content validity supported the ACSF's applicability, clarity, and usability. The ACSF adaptations did not change its original construct. Reliability were calculated from 90 parent and professional Time-1 and Time-2 ratings for children/youth (2.1-15.6 years). Results showed good-to-very good intra-rater agreement (typical) and good inter-rater agreement (capacity).