Is the difference between toAmount and toAmountMin purely based on the slippage and if this is the case which is the default slippage when calling /quote route?

Yes, the difference between toAmount and toAmountMin is purely based on slippage. 

• toAmount is the estimated post-swap amount
• toAmountMin is the guaranteed minimum amount based on your slippage setting
• Slippage is defined as the difference between toAmount and toAmountMin
  
  
  
  Default slippage is 0.5% (0.005) when no slippage parameter is provided in API requests. Users can set custom slippage by using the slippage parameter in /quote and /route endpoints. 
  For Solana same-chain swaps, omitting the slippage parameter enables auto-calculation based on on-chain market conditions. Users can use the default, enable auto-calculation (Solana), or specify exact tolerance levels.
  
  Here is a guidance from our main Website main site with additional details and AI-powered search. Visit here for the full experience
  https://docs.li.fi/guides/integration-tips/slippage-price-impact