You must comply with the summons and appear for questioning.
The distinction as to whether you have been summoned as a witness or as an accused is particularly important with regard to the legal position, as the position of the witness differs in certain aspects from that of the accused. For example, as a witness you are obliged to testify completely and truthfully, unless there is a right of refusal. If you are the accused, you have the right to refuse to testify.
As the accused, you should in any case consult a lawyer during the questioning. Statements made to the police can have a negative effect on the subsequent proceedings. A defense attorney can prepare you for the interrogation before it takes place and is present during the interrogation to ensure that your rights are respected.