In the case of fluorescence probes associated non-covalently with proteins, (for example porphryins, FAD, NADH or ANS to give but a few systems), the probe is held to the protein matrix by several points of attachment and hence its “local” mobility, that is, its ability to rotate independent of the overall “global” motion of the protein, is very restricted. 
In the case of a probe attached covalently to a protein, via a linkage through an amine or sulfhydryl groups for example, or in the case of tryptophan or tyrosine sidechains, considerable “local” motion of the fluorophore can occur.  In addition, the protein may consist of flexible domains which can rotate independent of the overall “global” protein rotation.  This type of mobility hierarchy is illustrated on the right for the case of a probe covalently attached to a dimeric protein
(a)
N
N
C
C
dye
(c)
(b)
Rotational Modalities
(a)  overall L7/L12 rotation
(b)  movement of one C-domain relative to other domains
(c)  movement of dye molecule around its point of attachment
Polarization IX