You start the game on Tier 1. Building a Tech Center (available in the outpost) unlocks Tech 2 units and technology (upgrades and morphs). Morphing up the starting Tech Center takes your army to Tier 3 (T3, for short), unlocking T3 technology. Morphing it again takes you to T4, and so on (up to T6)
A T4 tech center is required to morph the outpost to T4 or morph standard factories into advanced factories
A T4 outpost can build T4 factories and structures, including a T4 outpost directly (not requiring morphs)
If all upgraded Tech Centers (nothing prevents you from building many) are destroyed, that locks out some of the advanced units from the advanced factories and outposts, but not all of them