When pressing 'B' at the start of a round, your entire team needs to do exactly the same thing.
The Full Eco ($0 Spent): You buy absolutely nothing. You do this when your team has under $2,000 and a low loss bonus. The goal is simply to survive and plant the bomb for the extra $800 team bonus.
The Half-Buy (The $2,000 Rule): You have some money, but not enough for rifles. The goal of a Half-Buy is to spend just enough to be dangerous (upgraded pistols and armor), but ensure every player is guaranteed to have at least $2,000 remaining. If you save $2,000 and lose the round (minimum +$1,900 loss bonus), you are mathematically guaranteed to have $3,900 next round—exactly enough for an M4 or AK and armor.
The Force Buy: Spending every dollar you have to break the enemy's momentum, usually done when the enemy team’s economy is also fragile.