I am writing an article in which the experimental part was carried out according to the kit instructions; how could I cite it? (The kit file is attached)
I would have cited this not as the paper slip coming with some gear in a box, but as an Internet resource. It is this document, isn’t it? http://www.labindustrias.com/web/wp-content/uploads/2020/06/INS-DT4-EN-ichroma™-T4-Rev.02.pdf
it is a good practice to refer to a document that readers can access, rather to something unpublished or temporary.
for citing electronic online documents in APA style, check as example https://apastyle.apa.org/style-grammar-guidelines/references/examples/webpage-website-references or consult instructions for the reference system you will be using.
Abdounasser Albasher Omar Use the MLA style template to reference a primary-source document from a kit. Begin by providing the document's title or a description. Then, as the container's title, including the kit's title and any relevant publishing information: Illumination from a book of hours from the fourteenth century.