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The term escape fraction is used for the faction of ionizing photons (e.g., produced by stars) that escape from some object (despite any internal neutral atoms that could catch them). This includes extreme ultraviolet photons as well as any X-rays and gamma rays. Galaxies, for example, have such escape fractions, and the escaping photons can ionize neutral atomic hydrogen that surrounds the galaxy. Early stars produce much of such EMR within galaxies. Current modeling suggests such fractions range from a few percent to more than a half.
The escape fraction of high redshift galaxies is of interest in working out the history of reionization, e.g., whether (and which) galaxies provided the photons.