you can read / understand the most fundamental aspects of the subject area.
ability to implement small changes, understand basic principles and able to figure out additional details with minimal help.
basic proficiency in a subject area without relying on help.
you are comfortable with the subject area and all routine work on it:For software areas - ability to develop medium programs using all basic language features w/o book, awareness of more esoteric features (with book).For systems areas - understanding of many fundamentals of networking and systems administration, ability to run a small network of systems including recovery, debugging and nontrivial troubleshooting that relies on the knowledge of internals.
an even lower degree of reliance on reference materials. Deeper skills in a field or specific technology in the subject area.
ability to develop large programs and systems from scratch. Understanding of low level details and internals. Ability to design / deploy most large, distributed systems from scratch.
you understand and make use of most lesser known language features, technologies, and associated internals. Ability to automate significant amounts of systems administration.
deep understanding of corner cases, esoteric features, protocols and systems including "theory of operation". Demonstrated ability to design, deploy and own very critical or large infrastructure, build accompanying automation.
could have written the book about the subject area but didn't; works with standards committees on defining new standards and methodologies.
wrote the book on the subject area (there actually has to be a book). Recognized industry expert in the field, might have invented it.