A "Fixup" is a generalised action applied to a fully generated component tree on a render cycle that "fixes up" the component values by merging them with data taken from some source.
The main sort of fixup is the "EL value fixup". Any components of type UIBound (that includes all UIInput, UIOutput and nested instances in clusters such as UISelect) which do not have a value set, but which do have a value binding, will have the value filled in by querying the value binding.
A second sort of fixup is an "RSVC fixup", whereby the RequestSubmittedValueCache for the current request will be queried for any "unapplied" values that are yet to be applied to the model. These values may be in the RSVC as a result of a previous erroneous submission that is being returned to the user, or as a result of an RSVC replay state preservation strategy (not yet fully implemented). These cached values will take priority over any values either in the model, or supplied to UIBound objects if they are found during a render cycle.
In terms of the handler sequence diagram, fixups execute as "step 8" of the rendering pipeline, after constructing the view tree by a producer and before rendering via IKAT.