Update 2025 to the product availability listed below.
US Coatings is marketing a product very similar to BCS under the brand name Abatron, which for Canada is much easier to source than my old stand-by BCS. It is encouraging as a quality endorsement that LeeValley Tools is selling this brand, which tells me I can count on it as I have always counted on BCS.
LeeValley sells the basic Abatron “Kit” as well as the 2-part sealer and filler separately. You can also find it on Amazon.ca (search “Abatron wood restoration kit”) with more selection than LeeValley, but the same price. I would choose to support the Canadian company LeeValey rather than Amazon if the standard kit works for you. In addition, some of the extended Abatron product listings on Amazon are not fully supported by Amazon, indicating some customer problems, probably delivery.
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Jack wants to know just how a door frame is made, so that he can take his apart. The sill is rotten.
Usually, but not always, the sill goes right under the vertical framing. Sometimes the vertical piece is notched into the sill. In either case, you have to either cut out all the rotten wood, or treat any part that has active fungus. The good news is that it is actually possible now, with special epoxies, to rebuild the whole rotten area without removing much of anything.
Minwax makes a Wood Hardener and a ridgid wood filler while BCS and Abatron use 2-part liquid wood epoxy that you use to solidify the rotten wood and kill off the fungus. 
Then the BCS and Abatron systems go futurer with a 2-part epoxy putty that fills in any missing wood. These are often used for door and window sills, as well as for rebuilding the bottom of wooden columns and porch rails, much stronger than any single component filler putty.
A unique use for these is to repair rotten joists under toilets. The hardener from all three products will kill all the fungus and stop all changes in the wood allowing for sistering other joists to the side of the rotten ones. The 2-part epoxy products properly applied can even restore the origional joist to its previsous strength.
Follow this link for details and working techniques when working with these penetrating epoxies.