Non Redundant list:

Nonredundant (NR) datasets of 3D structures for RNA is a list of best resolved and modeled representatives of each structure class available for analysis. Redundant files in the RNA 3D structure database are identified and clustered into equivalence classes and a representative structure is listed for each class.

Equivalence classes:

Structures that are provisionally redundant based on sequence similarity and also geometrical similarity are grouped into one Equivalence class.

Leontis, N. B., & Zirbel, C. L. (2012). Nonredundant 3D Structure Datasets for RNA Knowledge Extraction and Benchmarking. In N. Leontis & E. Westhof (Eds.), (Vol. 27, pp. 281–298). Springer Berlin Heidelberg. doi:10.1007/978-3-642-25740-7_13

RNA 3D Motifs:

RNA 3D motif is a well­ defined geometric arrangement of interacting nucleotides. These motifs are identified from a non­redundant set of RNA- containing PDB files. These structures were compared, the loops were clustered and distinct motif groups were identified, including all hairpin and internal loop motifs previously described in the literature. For more information please go to RNA 3D Motif Atlas
The RNA 3D motif data is available only for those structures in NDB that make up the non redundant list.

Relative Frequency:

Relative Frequency is the number of observations of a given interaction type divided by the total number of observations of all the interactions.

Eg: If a structure had a total number of 50 Base pair interactions and a total of 25 Cis Watson-Crick/Watson-Crick (cWW) interaction among them. Then the relative Frequency of cWW in the structure is 25/50= 0.5