Capture the Flag battle mode, or CTF for short, is one of the most popular battle modes in Tanki X. It is, by default, a team-based mode, and hence, players are generally split into 2 teams - red and blue. Depending on your preference, you can join whichever team you like unless the balance between teams are grossly unbalanced, then you can only join the team with the least players.
As its name implies, the main goal in CTF is to capture the enemy's flag as many times as you can. To do so, a player in your team will need to travel all the way to the enemy's base where their flag will be; run the tank over the opposing team's flag to "pick it up"; and drive all the way back to your team's flag holder in one piece. Of course, while a player is doing that, the other players will either be assisting him or be defending your team's flag from being captured by the enemy.
Flags cannot be dropped once picked up, so generally, you can't pass a flag to another player unless you initiate the self-destruct sequence on your tank. Flags that are dropped will remain in their respective positions for awhile before resetting. You can also "touch" the flag with your tank to immediately reset the flag to its rightful position at your team's base.
The team with the most flags at the end of the day will win and gain a substantial amount of diamonds as rewards. However, each team only has a limited amount of time to accumulate the flags.