Which of the following is not a valid Insert Symbol option when the main manufacturer part has circuits associated with specific symbols?

The option "Use last inserted symbol" is not valid in the context of inserting symbols associated with specific manufacturer parts that have circuits tied to designated symbols. This is because integrating specific symbols requires acknowledging each symbol's unique characteristics and connections within the circuit.

Using the last inserted symbol does not take into account the requirements of the current part or its associated circuits. It might lead to inconsistencies or errors in the design since the last inserted symbol could be unsuitable or irrelevant for the new context. The function to utilize previous symbols might not apply when working with specialized components that necessitate precise symbol representation.

On the other hand, assigning a new symbol, inserting a custom symbol, and removing the current symbol are all valid operations that allow for precise control over the symbol selections that directly correspond to the specific circuits and parts being used. These options enable designers to ensure that the symbols represent the functionality and characteristics of the components accurately, thus maintaining the integrity of the electrical schematic.