To avoid the problems that the limit of the error tolerance approaches infinity as the mass approaches zero, or approaches zero as the mass becomes very large, it is necessary to place some additional limits on the errors at low and high mass. If the error tolerance is expressed in u or mmu then that is the only parameter needed to define the error limits for the elemental composition calculation. Each ring and each double bond counts as one site of unsaturation and each triple bound counts as two sites of unsaturation. Note that the calculation includes a division by two, so that the result can be either an exact integer, such as 4.0, or an integer with a remainder of 0.5. A neutral, un-ionized compound will have an even number of electrons.
Ions can have either an even or odd number of electrons.
The electronic configuration can be calculated for any given elemental composition by calculating the unsaturation. This information can be used to identify molecular ions or fragment ions and to identify fragment ions that are likely to result from rearrangement reaction. Most fragment ions in EI are formed by the loss of a radical, so most fragment ions have an even-electron configuration. This can be used to limit the output. If you are looking for molecular ions in an accurate mass analysis of EI data, limit the output to ODD-electron ions.