It is stated here that:
The Unix operating system introduced the idea of a toolkit. That is, rather than having a few generalized and complex utilities programs, the system contains a toolkit of small and simple programs which the user combines to perform sophisticated tasks and to create custom programs.
The author continues with an example where it mentions
So my question is:
"What requirements must a computer program meet to be considered as part to the unix toolkit?".
As stated in @Jesse_b's answer the POSIX standard defines a list of standard utilities. This list may change in the future. I would like to know what are the requirements that a utility must cope to be admited in such list.