A profile fragment is the corresponding identity value and data which is ingested as record or event data into a dataset. Please refer to the following article.
https://experienceleague.adobe.com/docs/experience-platform/profile/home.html?lang=en#profile-fragme...
The limitations in the in the guardrails document refer to data ingestion patterns. Guardrail limits are often met or exceeded when there are array based data structures used for data ingestion to the Adobe Realtime CDP platform.