I have been asked this question several times now and I find it hard really pinpoint myself. However, I think there is a few things to bear in mind. When talking about Specialist Teaching Assistant and an HLTA.
To start off with its important to understand that the employer have an overall say in what happens within your organisation in regards to Teaching Assistants job role, however, there has to be considerations in regards to workload and subject knowledge and how confident you feel. Advice can be found on the following website about this.
A HLTA is a teaching assistant with supervisor responsibilities over other teaching assistants and may plan and cover lessons as and when required.
They work with teachers and SENCOs to improve and raise standards of learning and wellbeing.
A level 3 TA has not got management tasks or supervisor responsibility for the other TAs they would normally have a subject they are confident in and would support and oversee projects to do with that subject e.g. English TA would attend all core skills meeting and work with the teacher closely. They may deliver planing from the teacher but they would not make lesson plans. However, they are allowed to contribute to lesson plans they may also ask have meetings and information.