Not necessarily. A summons can be synonymous with an invite although it is often more strongly worded. You can summon someone for tea at which point it would be a bit rude if they decline. So when the foreign office is summoning the Chinese ambassador they tell him the time and place of the meeting and expect him to be there. The content of the meeting or how it is performed is a separate issue. But typically when an ambassador is summoned in this way it is because the government have a few important and urgent questions or statements for the ambassador. Demanding that he stand like a school boy for the duration of the meeting would be an insult and imply that he is subordinate to the British government, which he is not. But you can discuss these things in a more civilized matter like equals, each a diplomat of their own country.