Time flows in a specific direction, otherwise known as times arrow, for most objects this is fixed (or rigid) and their existence has a fixed start point and end point, point of birth or creation and the point of death or destruction. This tendency for normal matter to follow a rigid point to point path through time is known as temporal rigidity. Time travel invariably involves breaking or bending this rigidity.
Fixed
Normal matter, occurring naturally, has a fixed temporal location, origin, and destination within the temporal domain. That is to say, they sit able to move within 3 dimensions dynamically but unable to alter the position along the timeline.
Unfixed
Unfixed matter has somehow been altered from its natural state, though it is possible this alteration happens through some natural means it is extraordinarily rare. Once altered matter is left without an anchor point within the temporal domain. That is to say it can shift either at will (for living creatures surviving such alteration) or randomly.
Total Absence of Anchor
When an object has a complete absence of an anchor within the temporal domain it can shift through space-time and across multiple realities within its own universe and potentially across barriers between universes. Being able to shift point in time, origin in time, destination in time, and reality would cause matter to be able to become integrated into any timeline altering that timeline by its presence.
Partial Anchor
Matter which exists with a partial anchoring in time would be able to shift in time from that anchor point, or points. For instance, if an object was anchored at its origin point (or some other temporal locus) it would not be able to become integrated into an entire timeline but could travel through time. Going from any point to any other point but always being tied to a single point in their reality and time of origin. Time travel works by creating a temporary fixed point to anchor from and then shifting, artificially, ones point in the temporal domain, still tied back to its origin.
Unfixed Destination
An object with an unfixed end-point within the temporal domain would have a shifting and unfixed reality. That is to say external forces could alter its reality and direction within the time-stream, having an unfixed or unpredictable series of events in the near and distant future. In these cases events leading up to a point in time would not necessarily be able to predict events following a point in time. With an unfixed destination in the temporal domain cause and effect could be wildly unpredictable.
Unfixed Origin
With an unfixed point of origin in the temporal stream an object would have a fixed point in the temporal domain it was going to end up but events going on would alter the objects past wildly. Effect would drive cause and potentially cause severe paradoxes relative to its location in time. With an unfixed origin, the origin point in the temporal domain would change and effect could alter cause moving back along the space-time continuum. Changes moving backward along the continuum could splinter existing pasts, realities, and cause havoc with a timeline.
Unfixed Reality
When the reality is unfixed in the temporal domain but origin and destination are fixed (or some combination there of) an object can shift its position between realities but the proverbial clock is always ticking. The objects location in time never changes just which time line it resides within changes.
Living Creatures
A living being unfixed in time would have a difficult time maintaining a grip on reality. Shifts between times and realities would be random unless the living creature could find some method (technological, supernatural, or natural) of controlling the transition between times. Such an act would require a great deal of power as well as physical and mental discipline. It is not currently known if a living, sentient, creature could survive when unfixed in time.