Refinement R Factors
Searches for structures in the specified resolution range. A histogram icon next to the search box links to a histogram that shows the distribution of resolution in the PDB.
Searches for structures in the specified R factor range. R factor is the measure of the agreement between the structural model and the experimental diffraction data. When solving the structure of a protein, the researcher first builds an atomic model and then calculates a simulated diffraction pattern based on that model. The R factor measures how well the simulated diffraction pattern matches the experimentallyobserved diffraction pattern. A totally random set of atoms will give an R factor of about 0.63, whereas a perfect fit would have a value of 0. Typical values are about 0.20. See the "Looking at Structures: Resolution" page (here) for more details.
Note: nondiffraction methods such as NMR do not have a resolution specified.
The R factor search supports four types of metrics:
 R observed  Residual factor R for reflections that satisfy the resolution and observation limits. This quantity includes reflections set aside for crossvalidation in a "free" Rfactor set. Residual factor R for reflections that satisfy the resolution and observation limits.
 R all  Residual factor R for all reflections that satisfy the limits established by high and low resolution cutoffs. It includes reflections which do not satisfy the sigma cutoff criteria as well as those set aside for crossvalidation in a "free" Rfactor set.
 R work  Residual factor R for reflections that satisfy the resolution and observation limits and that were used as the working reflections in the refinement. This quantity does not include reflections set aside for crossvalidation in a "free" Rfactor set.
 R free  Residual factor R for reflections that satisfy the resolution and observation limits and that were excluded from refinement to be used for cross validation in a "free" R factor set. R free is a less biased metric, and is typically higher than an R work. If R free is significantly higher than the R work, the structural model may be over fitted.
Examples

Search for structures with an R free values between 0.18 and 0.24
"R free": Enter "Between: 0.18 and 0.24

Search for structures with an R free value of up to and including 0.22.
"R free": Leave the left box empty and enter 0.22 into the right box