If the electrons striking the target eject an electron from the K-shell of the atom, a vacancy is crated in the K-shell. Instantly an electron from one of the outer most shell, say L-shell jumps to the K-shell, emitting an X-ray photon of energy equal to the energy difference among the two shells. Likewise, if an electron from the M-shell jumps to the K-shell, X-ray photon of high power is emitted. The X-ray photons emitted because of the jump of electron from the L, M, N shells to the K-shells gives K?, K?, K? lines of the K-series of the spectrum.   

If the electron striking the target ejects an electron from the L-shell of the target atom, an electron from the M, N ..... shells jumps to the L-shell so that X-rays photons of smaller power are emitted. These photons form the lesser energy emission. These photons form the L-type of the spectrum. In a similar way the formation of M type, N type etc. may be explained.