There is a basic obligation to declare worldwide assets and income earned worldwide, because at least to determine the respective tax rate, these factors are also required in Switzerland.
In practice, it can always be found that e.g. houses and condominiums located abroad are not specified in the Swiss tax return. In most cases, this is probably due to ignorance of the corresponding declaration obligation or due to incorrect information.
The same applies to foreign custody accounts and the interest and dividends earned from them. In such cases, depending on the double taxation agreement, the taxation right (if DBA is in place) often lies with the country of residence, which is often Switzerland for taxpayers with unlimited taxation in Switzerland. This is despite the fact that in many cases foreign withholding taxes have already been withheld before the dividends are paid in the foreign country.
In order to avoid double taxation, the crediting procedures in the country of residence and the reimbursement procedures in the foreign countries (source countries) must be observed.