Stage: Candidate
Construct: Mix(core, tag)
To tag specific information ("tag" here) to a root value ("core"), e.g. a length of the contents the core represents.
This merkleization step is simply: H(core, tag), with both inputs left-padded to to a full 32 bytes.
SSZ is consistent with merkleizing in these tags to the right: it avoids a kink of branch nodes in common proof encodings. I.e. the left-most contents value and its branch of witness nodes go straight to the root.
In the spec the following function definition is used when mixing in a number:
mix_in_num(root, num): Given a root and a num ("uint256" little-endian serialization) return hash(root ++ num) (the ++ is concatenation).