In The Unicode Standard, Modifier letters are discussed in  Chapter 7 Europe-I: Modern and Liturgical Scripts. Modifier letters are generally used next to another character in order to modify the usage of the other character in some way. They are not Combining Marks, they are base characters. They are used in transcription of language in other scripts to the Latin script and they are used in Latin and Cyrillic orthographies. Other scripts have modifier letters which are specific to that script.

Spacing Modifier letters were first encoded in The Unicode Standard version 1.0. Since that time a number of other Modifier letters have been encoded. Modifier letters are now encoded in the following blocks:

BlocksCharacter RangeAdded in Unicode VersionDocumentation
Spacing Modifier Letters 02B0..02FF 1.0  U02B0.pdf
Modifier Tone Letters A700..A71F 4.1  UA700.pdf
Phonetic Extensions 1D00..1D7F 4.0  U1D00.pdf
Phonetic Extensions Supplement 1D80..1DBF 4.1  U1D80.pdf

Subsequent to version 1.1, the following characters have been added to the Spacing Modifier Letters and Modifier Tone Letters blocks. Please see Unicode Status for the Latin script for Modifier letters in the Phonetic Extensions blocks.

CharactersUnicode VersionDocumentation
02DF 3.0  n1812.pdf L2/98-261
02EA..02EB 3.0  n1713r.rtf L2/98-090
02EC..02ED 3.0  n1845.pdf L2/98-299
02EE 3.0  n1817.htm L2/98-295
02EF..02FF 4.0  n2419.pdf L2/02-141
A700..A716 4.1  n2646.pdf L2/03-339,  n2626.pdf L2/03-317 and  n2312.pdf L2/01-025
A717..A71A 5.0  n2883.pdf L2/04-349R
A71B..A71F 5.1  n2945.pdf L2/05/097R

Documentation refers to  ISO Working Group and Unicode proposals