With the advent of the present and future spatial X-ray missions, it becomes crucial to model correctly the line spectrum of X-ray emitting/absorbing media. We have built a photoionization code, TITAN, solving the transfer of a thousand lines and of the continuum with the “Accelerated Lambda Iteration” method (ALI), which is most reliable for line transfer. We give some details about this method and a justification for its use as a complement to usual approximations (e.g., escape probability or two-stream) made at present in other codes. We show that the escape probability approximation leads to a wrong estimation of the emitted X-ray line intensities, especially in the soft X-ray range. The errors can exceed one order of magnitude in the case of thick media (Thomson thickness of the order of unity). It also happens, but for different reasons, in the case of moderately thin media (Thomson thickness of 0.001 to 0.1), characteristic of the Warm Absorber in Seyfert 1 or of the X-ray emitting medium in Seyfert 2. Using TITAN, new diagnostics based on He-like ion lines of the n=2 complex are proposed and some examples are presented here, concerning in particular the influence of the transfer method on the G ratio. We show an example of the influence of the direction on the emission/absorption spectrum. We also give insights about the influence of numerical methods for the computation of radiative transfer, and how to handle the ALI method in such problems where important gradients occur.