There is a little difference between PvP and PvE, but not much.
If you've completed the rank missions, you'll noticed you're told you've gained T2 and T3 clearance when you reach certain star levels of gear.
Rank and Level are somewhat related, as higher levels/ranks require higher clearances (which you get, in the process of following the missions), but T2 and T3 clearance does not require any certain level or rank.
If memory serves, you need a 3* sword, shield, armor, and helm to be allowed into T2, and 5* for T3.
In the Clockworks, being higher tiers allows you to run deeper, harder levels. Tier 2 clearance lets you go past Moorcraft Manor, and Tier 3 clearance lets you go past Emberlight.
Tier clearances are only used in PvP to try to match players of similar gear levels together, and only in Lockdown, because Blast Network doesn't use any gear.
Here's where it differs from PvE: in PvE, you need a full set (more or less) to get clearance, for PvP, a single piece of 2* or 4* gear is enough to bump your tier up to the next higher one.
A single 5* trinket, even, will put you in Tier 3 lockdown games, even if you're only wearing proto gear.
Hence, you'll find 0-1* in T1 LD, 2-3* in T2, and 4-5* in T3, so an ideal lockdown set is all 1*, 3*, or 5*.