Figure A
Do not run the custom rail cap continuously beyond the 4 x 4 post. Do not exceed 48" between 4 x 4 post. 

Figure B
The above view illustrates the correct assembly method. Views S, T, U, & V reveal the expansion & contraction clip installation.  
(View "W & X" top circle)
(View "Y & Z" bottom circle) 

VIEW S
Clip may be attached between the Custom Rail Cap and Custom rail trim molding. (pre-drilling (2) holes in ½" trim 1st) 

VIEW T
Clip may be attached on the bottom of ½" x 2½" Custom Trim Molding 

VIEW U
Clip may be flipped as shown to reduce the gap distance 

VIEW V
Clip may be mounded as shown above but some of the clip may be exposed. 

VIEW W
Stair Rails are normally shorter in length causing minimal expansion and contraction problems. By drilling & counter sinking the screws in the 4 x 4 post, the rail system can be fastened prior to the deck rail assembly as shown in views W, X, Y & Z (or you may use a pre-bend rail clip shown above) 

VIEW X
Note:
As shown in views "S" & "T" the clip may be sandwich between the Custom Rail Cap & The Custom trim Molding with following alterations; With a wood rasp you must file out the area where the clip meets the butt end of the Custom Rail Cap. This will reduce a potential expansion & contraction gap of ¼" to a more attractive 1/8"