The CommandAPI has extra support for vanilla Minecraft's /execute command, by allowing the CommandSender to be an instance of the ProxiedCommandSender class. This allows the CommandSender to contain two extra pieces of information: The "proxied sender" and the original sender.

Say we have a command which kills the sender of a command. This is easily implemented as follows:

CommandAPI.getInstance().register("killme", new LinkedHashMap<String, Argument>(), (sender, args) -> {
    if(sender instanceof Player) {
        Player player = (Player) sender;
        player.setHealth(0);
    }
});

But what if the sender of the command is not a player? By using Minecraft's /execute command, we could execute the command as any arbitrary entity, as shown with the command below:

/execute as @e[type=chicken] run killme

To handle this case, we can check if the sender is an instance of a ProxiedCommandSender, and kill the callee (the entity which is being 'forced' to run the command /killme)

CommandAPI.getInstance().register("killme", new LinkedHashMap<String, Argument>(), (sender, args) -> {
    if(sender instanceof Player) {
        Player player = (Player) sender;
        player.setHealth(0);
    } else if(sender instanceof ProxiedCommandSender) {

        //Get the proxy CommandSender object from the CommandSender
        ProxiedCommandSender proxy = (ProxiedCommandSender) sender;

        //Check if the callee is an Entity
        if(proxy.getCallee() instanceof Entity) {

            //If so, kill the entity
            Entity target = (Entity) proxy.getCallee();
            entity.setHealth(0);
        }
    }
});

This allows the command above to run successfully, killing all chickens it can find.