In the case of a psalm tone it would indicate the particular final cadence that is to be used. A number (most) of the psalm tones have anywhere from two or three to over a dozen differentia, final cadences, by which the tone's final cadence coincides with the respective antiphon.

Others may put this better than I have done.

I remember being told once that the difference between a Capital and lowercase referred to whether the final note of the psalm-tone was the "final" of the mode: So the Mode VIII tone ending on Sol is VIII-G, capitalized because G (Sol) is the "final" of Mode VIII; but the tone ending on Do is VIII-c, lowercase because c (Do) is not the final of Mode VIII. So we have the endings I-D, II-D, IV-E, VI-F, VIII-G, etc.; it is interesting to note that, with the sole exception of Mode I, only the plagal modes (II, IV, VI, VIII) have tones that end on their final. (Though I don't have a book with me at the moment to double check if that's 100% true.)

@Salieri true, but the more recent Solesmes books have abandonned this convention and the letters are printed all lowercase, both final and non-final.