In general terms protocol is a code of correct practice or a guide to behavior practice. It can be also the first copy of a treaty or other such document before its ratification (approval or confirmation).
From the information in the question:
A convention exists between Spain and Iceland to avoid double taxation and avoid or report tax evasion with respect to tax duty on income, profit and property. There's a protocol (code of practice) established in Madrid, January 2002
There are guidelines on tax evasion and reports, including protocol for management between a business division and a personal division.
(PDF file on guidelines on the processing of tax reports, June 2022)
https://www.revenue.ie/en/tax-professionals/tdm/compliance/s...
From the reference below:
The present convention is applied to taxes demanded on income and property for each one of the binding states, their political subdivisions and local authorities with their system of tax levy or collection.
Tax on income and property are considered with the profit or property surplus
value, including tax on income derived from the disposal of property, similar to tax on surplus value.
https://www.boe.es/buscar/act.php?id=BOE-A-2002-20192

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Note added at 12 hrs (2023-08-24 22:09:59 GMT)
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Tax on capital gains is connected with the surplus value such as tax on an asset with an increased value.