“Serious electric guitar players want an all-in-one effects processor without skimping on tone. The April 3rd, 2017 release of the new Line 6 Helix LT Modeling Electric Guitar Processor does just that. Because of its lower price than the original Helix Platform products, the new LT model comes at a time when working musicians need more of a break on price than ever.”
So basically what you get with the new Helix LT is signal modeling ( impulse responses etc) without the extras. The biggest thing is that the LT is the most affordable product in the typically really expensive Helix Platform (which has had huge success).