In Unlimited Blade Works, we eventually learn that Archer was summoned as Rin had a relic associated with him (i.e. the pendant Shiro keeps). In the recent anime adaptation, there is also some statement to the effect of "a relic is required to summon a servant".
Similar sentiment is expressed in the Fate route of the visual novel. On the last day, Saber tells Shiro that he must have been able to summon her as a result of having had Excalibur's sheath embedded in him (similar to how it was embedded in Irisviel in Fate/Zero):
To summon a heroic spirit, one needs a symbol connected with that spirit.
This seems in line with the other summonings we see, such as Waver's use of a cloak to summon Iskandar and the trouble the Einsberns take to retrieve the sheath so that Kiritsugu summons a good servant. Finally, some remark Lord El Melloi makes about having to find a relic last minute suggests that a relic is always required.
Is this ("a relic is always required for a successful summoning") correct? If so, did Rin have anything she knew to be a relic when she summoned Archer? (I feel like she didn't, but if a relic is required, I would have assumed that she'd be aware of this, as a reasonably competent magus.)