tr usually means the wire will have a stripe or dashed line of a different color, stands for tracer, the line can be any color other that the main color of the wire.
White would be solid white
White tr would be white but with a line of a different color along its length
tr was used because there may have been a wiring change that year or just in general over the years they didn't stick with a single tracer color.
"w"=with
"/" can also = with
white /red tr would mean white wire with red stripe
white w/tr would mean white wire with with colored stripe, color unknown
I think turn signals on Jeeps use light green wire for one side and light green with stripe(tr) for the other side
Ford wiring diagrams abbreviate both colors, like wht/red, this would be a white wire with red stripe
or blk/wht, this would be black wire with white stripe, primary color first, tracer color second