How to define drop-down values for a field dynamicaly?

Here are my two examples:

1) Dynamic picklist based on IBPL with constraint

A few of our projects have variants, We have implemented that as a boolean on the Project type to flag the items that are variants of another one.

For example, project "ABC" has variants "Var1" and "Var2", so we have three Project items, "ABC", "ABC/Var1", and "ABC/Var2". Note: the common root is "ABC" which we call the Program, so we know all three project items belong together.

Our issue tracking system is designed to assign Issues to Projects, but if the project has variants, then you can also refine that by specifying which variants are affected, if any.

On the Issue type, we use an IBPL called "Variants" that allows to pick any Project items flagged as a variant:

But we want to select only the variants that are relevant for the selected project, so we add a constraint on the Issue type. Note that the picklist we use for projects is not the "Project" field, but another one called "Project Link".  The constraint is:

Without going through all the details, the constraint essentially says "only select the variants that belong to the same Program but exclude the current project". For example, if our issue selects Project Link "ABC", then the two variants are shown. But if the Project Link is "ABC/Var1", then only the second variant is shown.

Examples:

2) Simple pick list with constraint

Our issues can be categorized, for example, if it's a Quality Issue, all possible values are defined here:

But on the Issue type, values are constrained based on some other field value used for further categorization:

I believe this answers the question "define drop-down values for a field dynamically?".

Note that no scripting (e.g. trigger) or "html code" are required.