If you add an object to a model while the Current Time pointer is at a position other than zero on the timeline, that object is assumed to have existed in that same position from time zero. This is the default for each object. However, you can decide whether or not it exists at any point in time by adding a Life attribute to the object.
The Life checkbox does not automatically appear as a property of an object unless you specifically add this attribute. To add a Life attribute to the selected object, select Add Attribute > Life Attribute command in the Plus menu. An entry is added to the Object Properties in the Project window.
The status of the Life checkbox indicates whether or not the object exists in the model at the time indicated by the Current Time pointer. Then, by changing the status of this checkbox at different times, an object can appear and disappear at any time during the course of an animation.
To remove a Life attribute, select the object, then select Delete Attribute > Life Attribute in the Plus menu of the Project window.