Formatting changes how a result reads without changing the calculation. A ratio stored as 0.25 needs a percentage format if readers should see 25%. Formatting it as currency would preserve its value while assigning the wrong meaning. Choose decimal places for the decision the reader has to make.
The Format pane separates shading, borders, lines and number formats. Worksheet shading fills the pane; header shading affects the labels around it. Grid lines help readers estimate position, but dense lines compete with a bar chart's lengths. Zero lines deserve separate treatment when a measure can be negative.
Dashboard formatting surrounds its objects. Use padding to keep neighboring sheets from running together and choose a background that leaves the marks legible. Titles and tooltips should name the unit and aggregation. A precise number with an ambiguous label still makes a poor report.