In April 2020 the UK Supreme Court had reason to discuss the difference between ‘significant’ and ‘substantial’ in the context of what is a ‘significant reduction in life expectancy’. The court said that ‘like the skin of a chameleon, the adjective takes a different colour so as to suit a different context’.
The word ‘significant’ often means something less than the word ‘substantial’. But in context it can mean ‘substantial’.
The court gave an example of the reduction of normal life expectancy of a 74 year-old to two years as not being significant whereas such a reduction for a 24 year-old would be a significant and therefore substantial reduction.
These words are often used in legal documents and care must be taken to use them correctly in the context. The fact that the meaning changes with context may be a reason not to use them at all and, if possible, be specific.