The impact factor is calculated using only citations that appear in journals that are indexed in Web of Knowledge. (In 2016, Web of Knowledge included approximately 12,000 journals.)
Only "citable items" are included in calculating the impact factor's denominator. Citable items include research articles and reviews. Other items published in journals (such as editorials, letters, news items, and meeting abstracts) are not included among the total of citable items, although any citations to them are included in the numerator. (The justification for this is that non-article items receive few citations, and ISI attempts to avoid depressing the impact factors of titles that include content like book reviews.)
As with any citation metric, the impact factor is not capable of accounting for the reasons for or significance of the counted citations—that is, it cannot distinguish between vital citations and incidental or critical ones. The impact factor of a publication outlets does not communicate any information about the quality or impact of any individual article.
There is some evidence that international journals are underrepresented, especially those that are written in languages besides English.
Because citation patterns differ dramatically among different disciplines, impact factors of journals in different subject areas should not be compared.