The basic premise of the RacialTech patch is to limit the research of specific techs to specific races. This is accomplished by modifying the TechTables.txt spreadsheet. In the TechApplications table, there is a column labeled 'Sort_Tag'. This column will normally contain a label such as 'Ship', or 'Region', or 'Achieve'. It may contain a tag called 'KEEPTHIS' - this tag ensures that the particular technology will always be researchable by every empire in the game.
The 'Sort_Tag' column supports multiple tags, separated by commas. It is this column that racial tags may be added to. The RacialTech.patch file is accompanied by a RaceAlias.ini.sample file. This file is fully functional, and should you choose to use it, you may remove the '.sample' from the name, and move the file into your main MoO3 directory.
As the name suggests, it is a list of aliases. There are two columns. The first contains a tag that matches the internal MoO3 executable format. For the playable races, it goes by the format 'RACEx00y', where x is the species value, and y is the race value. The species and race numbers can be obtained by looking at the first table in the RaceModifiers.txt spreadsheet. Note, there are three column headings of note, 'UISpecies', 'Species', and 'Race'. Ignore the 'UISpecies' column, it's not relevant.
The second column of the RaceAlias.ini file is the actual alias. You can set this to any value you wish (although only the first 8 characters are actually used, so ensure there are no duplications). You may then use that alias in TechTables.txt instead of using the RACEx00y value.
The alias is then added to the 'Sort_Tag' column of the tech you wish to limit to a specific race. You may add more than one race to a tech, should you want it to be shared.
Let's assume you want to change the Point Defense mount so that only Humans could research it. Using the default RaceAlias.ini file, the alias for Humans is, appropriately, 'Human'. The default 'Sort_Tag' for the Point Defense mount is 'Ship, Military'. To make it only researchable by Humans, you would change that to 'Ship, Military, Human'.
Now perhaps you decide that that's too limiting, and you want an Humanoid (that is, the species Humanoid) to be able to research it. Again, using the default RaceAlias.ini values, the 'Sort_Tag' would now be 'Ship, Military, Human, Evon, Psilon'. Instead of using aliases, you can use the actual RACEx00y values themselves. If you choose this route, you don't need the RaceAlias.ini file. You could set the 'Sort_Tag' to 'Ship, Military, RACE0000, RACE0001, RACE0002'. They would both have the same effect, but the former is certainly easier to read than the latter.
It is worth noting that the word 'research' has been used throughout this document. That is because the RacialTech patch only applies to reseach. The tech may still be obtained by other races by trading, stealing, or military action.