And for anyone who hasn't read it - The Inquisition encyclopedia contains a description of the Greco-numeric system which I've paraphrased below;
The psychic strength and ability range that runs from Alpha to Omega is called 'The Assignment'. The 'standard' range of psychic activity runs omega, psi, chi, phi, upsilon, tau, sigma, rho, pi, omicron, xi, nu, mu, lambda, kappa, iota, theta, eta, zeta, epsilon, delta, gamma, beta, alpha.
'Normal' human psychic ability is rho or pi. Ratings of omicron to kappa indicate low level residual psychic activity that the subject will probably not be aware of and is very hard to detect. Subjects with a rating of iota and up will manifest some psychic ability. Ratings of zeta and higher indicate a very powerful psychic talant. The top four ratings are found in only about one in a billion humans.
The Inquisition and Blackships are supposed to catch anyone with a rating of omicron or above, but in practise only kappa and higher are usually identified. Anyone rated iota must be immediately confined or executed. It is reckoned that human minds cannot manage any rating higher than beta and still remain sane.
That covers psykers, below the rating of pi we're into the region of blunts, individuals who are oblivious to psychic activity or probing. Their ability is graded too, for example an individual with a rating of upsilon might just be affected by a severe psychic approach but is otherwise inert. Omega rating indicates the far extreme, the so called untouchables who are so 'blunt' as to have a measurable anti-psychic effect.
The entry then goes on to talk about the 'alpha plus' ratings, which run alpha plus, beta plus, gamma plus etc. No rating higher than gamma plus has ever been measured and it is theorised that beyond zeta plus the mind would become a non-corporeal entity.