Caption numbers are simply SEQ fields and all fields have option switches which enable what you are trying to do. Where you need the A, B you will need a second field with a new sequence name.

Firstly put all the captions in normally. Each number will be a field with the following field code {Seq Figure}. You can see and edit the field code by selecting the figure number, right clicking and choosing Toggle Field Codes. Edit that text and then select and press F9 to refresh the field.

Each of these fields may also have a mergeformat switchbut that isn't necessary so I'll ignore those for the following explanation.

With a caption number that needs the letter A, you need to add a second field so the field codes are as follows
{Seq Figure}{Seq FigureLetter \* ALPHABETIC}

On the following caption you repeat this but add a switch to the first field so it repeats the previous number
{Seq Figure \c}{Seq FigureLetter \* ALPHABETIC}

Later if you need to restart another FigureLetter at A, you add switches to the fields like this
{Seq Figure}{Seq FigureLetter \r 1 \* ALPHABETIC}
{Seq Figure \c}{Seq FigureLetter \* ALPHABETIC}
{Seq Figure \c}{Seq FigureLetter \* ALPHABETIC}

Note that the seq field name you choose only has to be unique - 'FigureLetter' could have been any word other than Figure (although I wouldn't advise using 'Table' because that would likely be used elsewhere)

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Andrew Lockton
Chrysalis Design, Melbourne Australia