There are 3 types of Visitor:

• Standard Visitor: that are very easy to contain or do not need many resources to contain. It does not mean that the visitor is harmless, just simple to contain.
• Marriage/Civil partnership: that require more resources to contain completely or where containment isn’t always reliable. Usually this is because the relationship is insufficiently understood or is inherently unpredictable.
• DELETED.
• Transit visitor: that are very difficult to contain or haven’t yet been contained. Generally, the resources used to contain those are often very limited, yet they need much of the resources to contain it. This doesn’t mean the visitor is dangerous; just difficult to contain.

https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor