E-commerce model design terminology

A simple one. Given certain product, e.g. a shirt (definitions source here), if...

• An attribute is just a characteristic like color, size, type, etc.
• An option is each of the values associated with certain attribute, like red or blue (regarding to the color attribute).
• And a variation is each combination of attributes and each option of that attribute like, red shirt...

How can I denote a "property" defined by the user himself during the shopping process, like when he fills an input with a custom message to be written on the shirt?? Is it an option or not?

How is it normally modelled during database design for e-commerce solutions? Thanks.