I have a document dated 1992 (steam gauge Config) with some interesting notes on the SB and original SPI. From the Flight Procedures Handbook-Approach, Landing, and Rollout:
"SPI: ... The SB position (actual position from feedback SOP) and the SB command are displayed on the SPI. Prior to TAEM, the SB command displayed is the DAP auto command (Mach-scheduled SB command), and post TAEM, the SB command displayed on the SPI is the auto guidance SB command. The BFS SPI is no different from the PASS, except that the SB command displayed is driven to zero during A/L since there is no BFS A/L guidance."
So, reading that it would appear that the command mark on the SPI only displays 'auto' commands. But I don't think that is correct. In the same document, and description of the SB scale on the HUD is as follows:
"SB Scale - This This symbol displays the orbiter SB actual and commanded positions. ... Actual SB position is represented by a triangular point above the scale. The SB command (auto or Manual) is represented by an arrow pointer below the scale..."
I can't imagine they would have two displays for SB info (HUD & SPI) that have a identical way of displaying data (Scale with two pointers, one showing commanded SB and one for actual SB) but not function in the same way (the lower pointer on HUD shows auto and manual command while SPI only shows auto command). But the same as GLS's post, the sections never say that that is explicitly how the display functions. :shrug:
Again, this document is from 92 and pre-MEDS, but I don't think they would change the function of the SPI in the upgrade. If anything, the fact that the steam SPI is described the same way as in the SCOM (MEDS SPI?) would support GLS's conclusion. I can pass a long the document if anyone wants to take a look.