Tea horizon is broad, visible, reachable. Vertical tea is how deep one can reach.
My perception of exploring tea horizontally can be number of types, regions, varietals, processing, basically as many different teas as one can. Vertically can be substance within the leaves which is the essential to the quality, grades, climate influence, age, cultivation practise influence, one way to explore vertical depth is try everything in a region or even one tea of many versions.
Most of us explore horizontally at the beginning, which can be a lengthy process, some may never get to the vertical stage. In the vertical stage, one can learn the criteria of tea in depth, armed with this understanding, exploring tea horizontally in depth become clear and easy.