In Southern Standard British English, also know as RP, there are two highish, frontish unrounded vowels: the FLEECE and KIT vowels, /i:/ and /ɪ/. The former is the vowel heard in the word fleece, the latter the vowel heard in kit.
Speakers use one of these two vowels in certain weak, unstressed, open syllables (i.e. ones without a final consonant), for example in the last syllable of the word happy or the first syllable of delineate. Very old speakers favour /ɪ/ and younger speakers /i:/. John Well's innovation for the transcription of such words in dictionaries, and particularly for the benefit of EFL users, was to use the symbol /i/ for the vowel in such positions. It signifies that the vowel may be either a KIT or a FLEECE vowel. It does not signify a short vowel of the FLEECE quality, and it does not represent a third i-type phoneme in addition to KIT and FLEECE.
Here is what John Wells had to say in one of his blogs on this topic:
The symbol i does not mean “neither long nor short”. It means that RP traditionally has lax ɪ in these positions, but that many speakers nowadays use a tense vowel like iː. Therefore the EFL learner may use one or the other indifferently in these cases, because it does not make any difference whether the vowel is tense or lax. See further the discussion in LPD under "Neutralization" (p. 539 in the third edition).
In LPD I use the symbol i in those cases where some people have a tense vowel in place of the traditional RP lax vowel: namely, in weak positions that are
(a) word-final, as happy, coffee, valley,
(b) prevocalic, as various, euphoria,
(c) in the unstressed prefixes be-, de-, pre-, re- and certain word-like combining forms such as poly-.
The LPD there is the Longman Pronunciation Dictionary. John Wells was aware of the confusion this innovation caused and in several posts he mentions that "It seemed like a good idea at the time!"