Properly, I don't see any difference (if more knowledgeable people disagree — let me know!). Tolkien normally uses KALAT for an extended stem when talking about nouns, and TALAT for verbs, but that's trivially explicable, for KALAT is an actual noun stem 'a light' and TALAT a verbal 'to fall down'. The stems are discussed only in the súndokarme section of TQ, in PE18, so I don't believe you've missed anything.

They were discussed in PE18. From what I remember, the three basic stem forms are TA, TAL and TALAT: uni-, bi- and tri-consonantal stems. From these stems there are various basic transformation rules to produce Primitive Quenderin words: vowel prefixion and suffixion, vowel suppression in the middle of the word, vowel lengthening and strengthening, consonant lengthening and strengthening, nasal infixion adding derivational suffixes, etc. The resulting words were semantically connected to the root.

One variant form, TALTA, was so common it became considered its own stem form in the Common Eldarin period, and was the basis of new word formations.

KAL and KALAT and just bi- and tri-consonantal stems expressed using different consonants. AKLAT isn't one of the basic stem forms. Rather it is tri-consonantal stem with vowel prefixion and suppression.

Thank you +Paul Strack. I guess what confused me was K vs. T.

+James Coish Tolkien uses what pleases him more, quite interchangeably. However, there is no stem *KA, but AT/TA 'two' actually does, so we never find 'KA-stems' but always 'TA' for the reversibles.