Complex sentence: Form factors typically specify only the "internal" dimensions and layout of the case; but for rack-mounted and blade servers form factors may include precise "external" dimensions as well, since these cases must themselves fit in specific enclosures.
Split into simpler sentences: Form factors typically specify only the "internal" dimensions and layout of the case. For rack-mounted and blade servers form factors may include precise "external" dimensions as well, since these cases must themselves fit in specific enclosures.