JCTVC-L1003_v17 introduced a naming conflict by renaming RIdx to StRpsIdx, which happens to be an existing variable name.

In 7.4.6.1,

The variable StRpsIdx is derived as follows:
– If short_term_ref_pic_set_sps_flag is equal to 1, StRpsIdx is set equal to short_term_ref_pic_set_idx.
– Otherwise, StRpsIdx is set equal to num_short_term_ref_pic_sets.

In 7.4.7,

The variable StRpsIdx is derived as follows.
StRpsIdx = idxRps − ( delta_idx_minus1 + 1 ) (7-42)

Furthermore, in 8.3.2, there is this note:

NOTE 2 – A value of StRpsIdx in the range of 0 to num_short_term_ref_pic_sets − 1, inclusive, indicates that a short-term reference picture set from the active SPS is being used, where StRpsIdx is the index of the short-term reference picture set to the list of short-term reference picture sets in the order in which they are signalled in the SPS. StRpsIdx equal to num_short_term_ref_pic_sets indicates that a short-term reference picture set explicitly signalled in the slice header is being used.