When a macro is called, all its steps are executed sequentially.

However, you can define conditional steps - steps which are only executed if a condition is met. In order to do this, simply give the step a condition property, like in Chase - Double speed:

<step condition="Math.IsEqual(Playbacks.Editor.Times.ChaseSpeed, 0.0)">
  ActionScript.SetProperty.Float("Playbacks.Editor.Times.ChaseSpeed", 1.0)
</step>

This step is only executed if its condition is met - and the condition is a function written in double quotes: Math.IsEqual takes two values as arguments and returns true if both values are equal. In total, if the property Playbacks.Editor.Times.ChaseSpeed equals 0.0 then this step is performed and sets this property to 1.0.

Another mechanism is also related to control structures albeit strictly it is just juggling with booleans: simple toggle logic, see Timecode - Enable/Disable:

<step>ActionScript.SetProperty.Boolean("Timecode.Enabled", !Timecode.Enabled)</step>

This simply negates a variable, effectively turning it into its reciprocal value: if the variable 'Timecode.Enabled' is true it will be set to false, and vice versa.