What is the seasonal seal (lock)?

At the start of each season a seasonal seal levels the playing field for new players and keeps things fresh for veterans, by temporarily limiting higher-tier units and gear bonuses:

• Blue Lock: the first two weeks, only blue-tier units, armor bonuses and crafted item bonuses disabled.
• Purple Lock: the next 2 to 3 weeks, purple-tier units allowed, armor bonuses active but crafted bonuses still disabled.
• Golden Phase: after Purple Lock everything unlocks, full gold-tier units, armor bonuses and crafted bonuses for the rest of the season.