Proto Shield is Proto Man's shield, which he uses to compensate for his fragility.
In the Mega Man games, he has always had the Proto Shield, although he did not use it actively on the NES or Game Boy games to defend himself. In Mega Man 7, the Proto Shield appears as Proto Man's defense against projectiles during his friendly boss battle and an unlockable weapon for Mega Man to despawn some enemy projectiles while he remains still in the floor. In Mega Man Powered Up, Proto Shield protects Proto Man against projectiles, but the shield flies away when touching a projectile, although Proto Man will always have it again after starting or restarting a level. Proto Man's gameplay in Mega Man 9 and Mega Man 10 has him use his shield as a way to reflect enemy projectiles. In Mega Man 9 his shield was smaller, but in Mega Man 10, its size was increased to allow more reflection of enemy projectiles.
Since Proto Man's gameplay in Mega Man Maker is taken from Mega Man 9, the Proto Shield takes its reflecting skill. This function of the Proto Shield makes him able to attack even if he is equipping a weapon without energy or has Nothing as a weapon. It can also destroy Weapon Blocks if he is equipping the weapon necessary to destroy them. This includes weapons otherwise unable to destroy those blocks, like the Support Items or the Super Arrow, but Weapon Blocks of Nothing are still indestructible. Destroying such blocks that way does not require weapon energy, which makes it an alternative way to destroy those blocks without wasting energy. The Proto Shield's hitbox and size is taken from Mega Man 10 to allow Proto Man to take more advantage of his shield.
Non-reflectable projectiles originally were destroyed upon contact with Proto man's shield, but this was not carried over in Mega Man Maker.