Semi-customs allow for almost entirely free reign with deciding your Rixixi's genes. They typically can be obtained via sales and events, but it's possible to get one through Euphoria chasmjumps and Lucida if you're lucky.
A basic Semi-custom allows 4 Common Markings, 2 Uncommon Markings, and 2 Common-Uncommon Mutations. You can swap out the 2 Mutations for 1 Combination Mutation if desired! You do not have to use all the genes listed. All Semi-customs should allow at least this much. Non-passable genes do not count under markings or mutations.
Semi-customs do not have any time-based restrictions. They do not expire, and if you don't use them immediately, you can use any genes that come out between the time you get your semi-custom and the time you use it - including subspecies if you have that addon.
You do not have to decide your genes until design approval, and due to this, lineage is added at the time of upload. There will be one ancestor in the first line of the lineage and one in the second line. Geno-editing items are allowed - see the geno-editing item section below for the few restrictions.
Some semi-customs allow use of a subspecies, and should say so outright, for example, your semi-custom proof might say:
"Semi-Custom (Subspecies Allowed, 4 Common Markings, 2 Uncommon Markings, 2 Mutations)"
"You can use this comment as proof for your semi when posting in design approval! +Subspecies confirmed!"
As long as no specific subspecies is specified, this can be used for any non-standard subspecies, including ones released after you obtained your semi-custom. If the comment does specify what subspecies is allowed, you can only use that subspecies, even if more subspecies are released after that.
There aren't many restrictions on geno-editing items, but there are some things to note:
Charged Fogshard Piece - Chrono/Supercharged Fogshard Piece - Chrono/Charged Liquid Diamond Shard: If more than 10 genes are added, the lineage will have an additional ancestor in the last line.
Supercharged Fogshard Piece - Alchemy: Can only be used after upload. Get the semi's genes set in stone before swapping them. Charged Alchemy can be used during design approval.
Charged Fogshard Piece - Swap/Supercharged Fogshard Piece - Swap: Cannot be used due to the same ruling as Supercharged Alchemy - Swap shards cannot be used after upload.