Some dictionaries such as Cambridge use the letter 't̬', the International Phonetic Alphabet character meaning [t] but voiced, i.e. [d], for an instance of /t/ that may undergo flapping in North American English. This is not a problem for /t/ that is pronounced consistently the same as /d/ in the same environment, as in latter and ladder, because /t/ and /d/ here are pronounced identically as a tap (aka flap; IPA: [ɾ]). Some other dictionaries therefore use 'd', because the difference between /t/ and /d/ is neutralized in these contexts.†
Flapping of /t/ is almost always present after a stressed syllable, as in notice, protestant, etc., in North American English, but not necessarily so in other environments. In positive, ability, monitor, etc., /t/ may not be pronounced as [ɾ] as often as in the aforementioned words. But dictionaries which use 't̬' do often use 't̬' for /t/ in these environments too, so for these words 't̬' is better understood as a shorthand for "/t/ that may optionally be pronounced as [ɾ], but not always".
Adding to the confusion, these dictionaries also use 't̬' for /t/ in words like winter, which may be pronounced the same as winner in North America. But winter never rhymes with hinder, nor does center with gender.†† What they should indicate instead, in my opinion, is the fact that /t/ in an intervocalic /nt/ cluster may be omitted, not voiced. Some other dictionaries therefore transcribe, quite rightly in my view, e.g. winter as /ˈwɪn(t)ər/. (But note the /nt/-cluster reduction isn't quite as ubiquitous as the flapping of /t/ following stress.)
TL;DR: So in dictionaries like Cambridge, the American pronunciation notations for words like mental, /ˈmen.t̬ᵊl/, are better understood as "either [ˈmen.tᵊl] or [ˈmen.ᵊl]",††† even though /ˈlæt̬.ɚ/ means always the same as /ˈlæd.ɚ/ (phonetically [ˈlæɾ.ɚ]), and /ˈpɑː.zə.t̬ɪv/ means "either [ˈpɑː.zə.tɪv] or [ˈpɑː.zə.ɾɪv]".
† Even the author of a pronunciation dictionary that uses 't̬' approves of the use of 'd' for flapped /t/ (see the 23 Sept '08 entry).
†† There are a handful of exceptions: seventy, ninety, and carpenter, in which /nt/ may be pronounced not just with [nt] or [ɾ] but also with [nd]. I am not aware, however, of such words other than these three.
††† To be precise, the result of the /nt/ reduction is usually a nasalized tap [ɾ̃] (a shorter [n]), but /n/ in the same context (as in winner) is also often [ɾ̃], so I found it would be too finicky to make a distinction between [n] and [ɾ̃] here. See the Wikipedia article for more.