wip: write about accessors

docs/core/eventdata/tracks.md

@@ -297,8 +297,40 @@ and the innermost track state becomes directly accessible via
 
 ### Component sharing
 
+{class}`Acts::MultiTrajectory` is designed so that components can be shared
+between components. This can be achieved using the
+{func}`Acts::TrackStateProxy::shareFrom` can be used to set this up.
+
+Shareable components are
+- predicted parameters and covariance
+- filtered parameters and covariance
+- smoothed parameters and covariance
+- jacobian
+
+To illustrate why this can be useful, consider again {numref}`ckf_tree`, where
+$S_2$ and $S_3$ branch out from a shared $S_1$. In this case, the predicted
+parameter vector and covariance, as well as the jacobian from $S_1\to S_2$ and
+$S_1 \to S_3$ will be identical. In this case, the combinatorial track finding
+will use the sharing functionality to share these components.
+
+:::{attention}
+Sharing these components introduces *cross-talk* between track states, and this
+is intentional.  If e.g. the predicted covariance is modified through either of
+the track states, the changes will be visible when accessed from the other
+track state as well.
+:::
+
 ## Dynamic columns
 
+Aside from the static properties that both the track states and the track, the
+EDM supports adding almost arbitrary additional information as dynamic columns.
+The implementation of the dynamic column mechanism is given by the backend,
+where the interface layer classes {class}`Acts::MultiTrajectory` and
+{class}`Acts::TrackContainer` and associated proxies only coordinate the
+creation, access and copying of dynamic columns. The following illustrates the
+usage of dynamic columns for {class}`Acts::TrackContainer`, but usage on
+{class}`Acts::MultiTrajectory` is identical.
+
 (edm_track_accessors)=
 ## Accessors